ispmail-workaround-org/dist/ispmail-trixie/relaying/index.html
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</script> <ul class="top-level astro-3ii7xxms"> <li class="astro-3ii7xxms"> <details open class="astro-3ii7xxms"> <sl-sidebar-restore data-index="0"></sl-sidebar-restore> <summary class="astro-3ii7xxms"> <span class="group-label astro-3ii7xxms"> <span class="large astro-3ii7xxms">ISPmail for Debian 13</span> </span> <svg aria-hidden="true" class="caret astro-3ii7xxms astro-c6vsoqas" width="16" height="16" viewBox="0 0 24 24" fill="currentColor" style="--sl-icon-size: 1.25rem;"><path d="m14.83 11.29-4.24-4.24a1 1 0 1 0-1.42 1.41L12.71 12l-3.54 3.54a1 1 0 0 0 0 1.41 1 1 0 0 0 .71.29 1 1 0 0 0 .71-.29l4.24-4.24a1.002 1.002 0 0 0 0-1.42Z"/></svg> </summary> <ul class="astro-3ii7xxms"> <li class="astro-3ii7xxms"> <a href="/ispmail-trixie/" aria-current="false" class="astro-3ii7xxms"> <span class="astro-3ii7xxms">Start here</span> </a> </li><li class="astro-3ii7xxms"> <a href="/ispmail-trixie/upgrading/" aria-current="false" class="astro-3ii7xxms"> <span class="astro-3ii7xxms">Upgrading from Debian Bookworm</span> </a> </li><li class="astro-3ii7xxms"> <a href="/ispmail-trixie/install-debian/" aria-current="false" class="astro-3ii7xxms"> <span class="astro-3ii7xxms">Installing Debian</span> </a> </li><li class="astro-3ii7xxms"> <a href="/ispmail-trixie/install-the-software-packages/" aria-current="false" class="astro-3ii7xxms"> <span class="astro-3ii7xxms">Install software packages</span> </a> </li><li class="astro-3ii7xxms"> <a href="/ispmail-trixie/virtual-domains/" aria-current="false" class="astro-3ii7xxms"> <span class="astro-3ii7xxms">Virtual domains</span> </a> </li><li class="astro-3ii7xxms"> <a href="/ispmail-trixie/database/" aria-current="false" class="astro-3ii7xxms"> <span class="astro-3ii7xxms">Database setup</span> </a> </li><li class="astro-3ii7xxms"> <a href="/ispmail-trixie/overview/" aria-current="false" class="astro-3ii7xxms"> <span class="astro-3ii7xxms">Big picture</span> </a> </li><li class="astro-3ii7xxms"> <a href="/ispmail-trixie/dns-records/" aria-current="false" class="astro-3ii7xxms"> <span class="astro-3ii7xxms">DNS records</span> </a> </li><li class="astro-3ii7xxms"> <a href="/ispmail-trixie/tls-certificate/" aria-current="false" class="astro-3ii7xxms"> <span class="astro-3ii7xxms">TLS certificate</span> </a> </li><li class="astro-3ii7xxms"> <a href="/ispmail-trixie/postfix/" aria-current="false" class="astro-3ii7xxms"> <span class="astro-3ii7xxms">Postfix</span> </a> </li><li class="astro-3ii7xxms"> <a href="/ispmail-trixie/dovecot/" aria-current="false" class="astro-3ii7xxms"> <span class="astro-3ii7xxms">Dovecot</span> </a> </li><li class="astro-3ii7xxms"> <a href="/ispmail-trixie/lmtp/" aria-current="false" class="astro-3ii7xxms"> <span class="astro-3ii7xxms">LMTP</span> </a> </li><li class="astro-3ii7xxms"> <a href="/ispmail-trixie/webmail/" aria-current="false" class="astro-3ii7xxms"> <span class="astro-3ii7xxms">Webmail using Roundcube</span> </a> </li><li class="astro-3ii7xxms"> <a href="/ispmail-trixie/relaying/" aria-current="page" class="astro-3ii7xxms"> <span class="astro-3ii7xxms">Sending / Relaying</span> </a> </li><li class="astro-3ii7xxms"> <a href="/ispmail-trixie/imap/" aria-current="false" class="astro-3ii7xxms"> <span class="astro-3ii7xxms">IMAP</span> </a> </li><li class="astro-3ii7xxms"> <a href="/ispmail-trixie/managing-users-aliases-and-domains/" aria-current="false" class="astro-3ii7xxms"> <span class="astro-3ii7xxms">Managing users, aliases and domains</span> </a> </li><li class="astro-3ii7xxms"> <a href="/ispmail-trixie/catching-spam-with-rspamd/" aria-current="false" class="astro-3ii7xxms"> <span class="astro-3ii7xxms">Catching spam with rspamd</span> </a> </li><li class="astro-3ii7xxms"> <a href="/ispmail-trixie/anti-spoofing-dkim-spf/" aria-current="false" class="astro-3ii7xxms"> <span class="astro-3ii7xxms">Prevent spoofing using DKIM and SPF</span> </a> </li><li class="astro-3ii7xxms"> <a href="/ispmail-trixie/catch-all/" aria-current="false" class="astro-3ii7xxms"> <span class="astro-3ii7xxms">Catch-all addresses</span> </a> </li><li class="astro-3ii7xxms"> <a href="/ispmail-trixie/quotas/" aria-current="false" class="astro-3ii7xxms"> <span class="astro-3ii7xxms">Quotas</span> </a> </li><li class="astro-3ii7xxms"> <a href="/ispmail-trixie/going-live/" aria-current="false" class="astro-3ii7xxms"> <span class="astro-3ii7xxms">Going live</span> </a> </li><li class="astro-3ii7xxms"> <a href="/ispmail-trixie/automated-installation/" aria-current="false" class="astro-3ii7xxms"> <span class="astro-3ii7xxms">Automated installation</span> </a> </li> </ul> </details> </li><li class="astro-3ii7xxms"> <details open class="astro-3ii7xxms"> <sl-sidebar-restore data-index="1"></sl-sidebar-restore> <summary class="astro-3ii7xxms"> <span class="group-label astro-3ii7xxms"> <span class="large astro-3ii7xxms">ISPmail for Debian 12</span> </span> <svg aria-hidden="true" class="caret astro-3ii7xxms astro-c6vsoqas" width="16" height="16" viewBox="0 0 24 24" fill="currentColor" style="--sl-icon-size: 1.25rem;"><path d="m14.83 11.29-4.24-4.24a1 1 0 1 0-1.42 1.41L12.71 12l-3.54 3.54a1 1 0 0 0 0 1.41 1 1 0 0 0 .71.29 1 1 0 0 0 .71-.29l4.24-4.24a1.002 1.002 0 0 0 0-1.42Z"/></svg> </summary> <ul class="astro-3ii7xxms"> <li class="astro-3ii7xxms"> <a href="/ispmail-bookworm/" aria-current="false" class="astro-3ii7xxms"> <span class="astro-3ii7xxms">Start here</span> </a> </li><li class="astro-3ii7xxms"> <a href="/ispmail-bookworm/whats-new/" aria-current="false" class="astro-3ii7xxms"> <span class="astro-3ii7xxms">What&#39;s new</span> </a> </li><li class="astro-3ii7xxms"> <a href="/ispmail-bookworm/migrating-from-a-bullseye-to-a-bookworm-server/" aria-current="false" class="astro-3ii7xxms"> <span class="astro-3ii7xxms">Migrating from your old (Bullseye) server</span> </a> </li><li class="astro-3ii7xxms"> <a href="/ispmail-bookworm/big-picture/" aria-current="false" class="astro-3ii7xxms"> <span class="astro-3ii7xxms">The big picture</span> </a> </li><li class="astro-3ii7xxms"> <a href="/ispmail-bookworm/types-of-email-domains/" aria-current="false" class="astro-3ii7xxms"> <span class="astro-3ii7xxms">Types of email domains</span> </a> </li><li class="astro-3ii7xxms"> <a href="/ispmail-bookworm/install-debian-bookworm-on-your-server/" aria-current="false" class="astro-3ii7xxms"> <span class="astro-3ii7xxms">Installing Debian</span> </a> </li><li class="astro-3ii7xxms"> <a href="/ispmail-bookworm/install-the-software-packages/" aria-current="false" class="astro-3ii7xxms"> <span class="astro-3ii7xxms">Install software packages</span> </a> </li><li class="astro-3ii7xxms"> <a href="/ispmail-bookworm/creating-a-tls-encryption-key-and-certificate/" aria-current="false" class="astro-3ii7xxms"> <span class="astro-3ii7xxms">TLS certificate</span> </a> </li><li class="astro-3ii7xxms"> <a href="/ispmail-bookworm/prepare-the-database/" aria-current="false" class="astro-3ii7xxms"> <span class="astro-3ii7xxms">Preparing the database</span> </a> </li><li class="astro-3ii7xxms"> <a href="/ispmail-bookworm/making-postfix-get-its-information-from-the-mariadb-database/" aria-current="false" class="astro-3ii7xxms"> <span class="astro-3ii7xxms">Let Postfix access MariaDB</span> </a> </li><li class="astro-3ii7xxms"> <a href="/ispmail-bookworm/catchall-aliases/" aria-current="false" class="astro-3ii7xxms"> <span class="astro-3ii7xxms">Catch-all aliases</span> </a> </li><li class="astro-3ii7xxms"> <a href="/ispmail-bookworm/setting-up-dovecot/" aria-current="false" class="astro-3ii7xxms"> <span class="astro-3ii7xxms">Setting up Dovecot</span> </a> </li><li class="astro-3ii7xxms"> <a href="/ispmail-bookworm/let-postfix-send-emails-to-dovecot/" aria-current="false" class="astro-3ii7xxms"> <span class="astro-3ii7xxms">Let Postfix send emails to Dovecot</span> </a> </li><li class="astro-3ii7xxms"> <a href="/ispmail-bookworm/quotas/" aria-current="false" class="astro-3ii7xxms"> <span class="astro-3ii7xxms">Quotas</span> </a> </li><li class="astro-3ii7xxms"> <a href="/ispmail-bookworm/testing-imap/" aria-current="false" class="astro-3ii7xxms"> <span class="astro-3ii7xxms">Testing IMAP</span> </a> </li><li class="astro-3ii7xxms"> <a href="/ispmail-bookworm/webmail-using-roundcube/" aria-current="false" class="astro-3ii7xxms"> <span class="astro-3ii7xxms">Webmail using Roundcube</span> </a> </li><li class="astro-3ii7xxms"> <a href="/ispmail-bookworm/testing-email-delivery/" aria-current="false" class="astro-3ii7xxms"> <span class="astro-3ii7xxms">Testing email delivery</span> </a> </li><li class="astro-3ii7xxms"> <a href="/ispmail-bookworm/relaying-outgoing-emails-through-postfix/" aria-current="false" class="astro-3ii7xxms"> <span class="astro-3ii7xxms">Relaying outgoing emails through Postfix</span> </a> </li><li class="astro-3ii7xxms"> <a href="/ispmail-bookworm/catching-spam-with-rspamd/" aria-current="false" class="astro-3ii7xxms"> <span class="astro-3ii7xxms">Catching spam with rspamd</span> </a> </li><li class="astro-3ii7xxms"> <a href="/ispmail-bookworm/setting-dns-records/" aria-current="false" class="astro-3ii7xxms"> <span class="astro-3ii7xxms">Setting DNS records</span> </a> </li><li class="astro-3ii7xxms"> <a href="/ispmail-bookworm/prevent-spoofing-using-dkim/" aria-current="false" class="astro-3ii7xxms"> <span class="astro-3ii7xxms">Prevent spoofing using DKIM</span> </a> </li><li class="astro-3ii7xxms"> <a href="/ispmail-bookworm/managing-users-aliases-and-domains/" aria-current="false" class="astro-3ii7xxms"> <span class="astro-3ii7xxms">Managing users, aliases and domains</span> </a> </li><li class="astro-3ii7xxms"> <a href="/ispmail-bookworm/monitoring-and-backup/" aria-current="false" class="astro-3ii7xxms"> <span class="astro-3ii7xxms">Monitoring and Backup</span> </a> </li><li class="astro-3ii7xxms"> <a href="/ispmail-bookworm/automatic-installation-with-ansible/" aria-current="false" class="astro-3ii7xxms"> <span class="astro-3ii7xxms">Automatic installation with Ansible</span> </a> </li><li class="astro-3ii7xxms"> <a href="/ispmail-bookworm/success-stories/" aria-current="false" class="astro-3ii7xxms"> <span class="astro-3ii7xxms">Success stories</span> </a> </li><li class="astro-3ii7xxms"> <a href="/ispmail-bookworm/faq-frequently-asked-questions/" aria-current="false" class="astro-3ii7xxms"> <span class="astro-3ii7xxms">FAQ (frequently asked questions)</span> </a> </li> </ul> </details> </li><li class="astro-3ii7xxms"> <details open class="astro-3ii7xxms"> <sl-sidebar-restore data-index="2"></sl-sidebar-restore> <summary class="astro-3ii7xxms"> <span class="group-label astro-3ii7xxms"> <span class="large astro-3ii7xxms">Misc articles</span> </span> <svg aria-hidden="true" class="caret astro-3ii7xxms astro-c6vsoqas" width="16" height="16" viewBox="0 0 24 24" fill="currentColor" style="--sl-icon-size: 1.25rem;"><path d="m14.83 11.29-4.24-4.24a1 1 0 1 0-1.42 1.41L12.71 12l-3.54 3.54a1 1 0 0 0 0 1.41 1 1 0 0 0 .71.29 1 1 0 0 0 .71-.29l4.24-4.24a1.002 1.002 0 0 0 0-1.42Z"/></svg> </summary> <ul class="astro-3ii7xxms"> <li class="astro-3ii7xxms"> <a href="/article/updating-the-bios-on-lenovo-laptops-from-linux-using-a-usb-flash-stick/" aria-current="false" class="astro-3ii7xxms"> <span class="astro-3ii7xxms">Updating the BIOS on Lenovo laptops from Linux using a USB flash stick</span> </a> </li><li class="astro-3ii7xxms"> <a href="/bacula-cheatsheet/" aria-current="false" class="astro-3ii7xxms"> <span class="astro-3ii7xxms">Bareos/Bacula Cheat Sheet</span> </a> </li><li class="astro-3ii7xxms"> <a href="/debian-packages-are-so-old/" aria-current="false" class="astro-3ii7xxms"> <span class="astro-3ii7xxms">Debian packages are so old</span> </a> </li><li class="astro-3ii7xxms"> <a href="/getting-help-on-irc/" aria-current="false" class="astro-3ii7xxms"> <span class="astro-3ii7xxms">Getting help on IRC</span> </a> </li><li class="astro-3ii7xxms"> 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0;">Protecting against forged sender addresses</span> </a> </li><li class="astro-g2bywc46" style="--depth: 0;"> <a href="#test-it" class="astro-g2bywc46" style="--depth: 0;"> <span class="astro-g2bywc46" style="--depth: 0;">Test it</span> </a> <ul class="isMobile astro-g2bywc46" style="--depth: 1;"> <li class="astro-g2bywc46" style="--depth: 1;"> <a href="#no-encryption-no-authentication" class="astro-g2bywc46" style="--depth: 1;"> <span class="astro-g2bywc46" style="--depth: 1;">No encryption, no authentication</span> </a> </li><li class="astro-g2bywc46" style="--depth: 1;"> <a href="#with-encryption-no-authentication" class="astro-g2bywc46" style="--depth: 1;"> <span class="astro-g2bywc46" style="--depth: 1;">With encryption, no authentication</span> </a> </li><li class="astro-g2bywc46" style="--depth: 1;"> <a href="#with-encryption-and-authentication" class="astro-g2bywc46" style="--depth: 1;"> <span class="astro-g2bywc46" style="--depth: 1;">With encryption and authentication</span> </a> </li> </ul> </li><li class="astro-g2bywc46" style="--depth: 0;"> <a href="#gory-details-of-authentication" class="astro-g2bywc46" style="--depth: 0;"> <span class="astro-g2bywc46" style="--depth: 0;">Gory details of authentication</span> </a> </li><li class="astro-g2bywc46" style="--depth: 0;"> <a href="#footnote-label" class="astro-g2bywc46" style="--depth: 0;"> <span class="astro-g2bywc46" style="--depth: 0;">Footnotes</span> </a> </li> </ul> </div></details></nav></mobile-starlight-toc><script type="module" src="/_astro/MobileTableOfContents.astro_astro_type_script_index_0_lang.C181hMzK.js"></script></div><div class="right-sidebar-panel sl-hidden lg:sl-block astro-pb3aqygn"><div class="sl-container astro-pb3aqygn"><starlight-toc data-min-h="2" data-max-h="3"><nav aria-labelledby="starlight__on-this-page"><h2 id="starlight__on-this-page">On this page</h2><ul class="astro-g2bywc46" style="--depth: 0;"> <li class="astro-g2bywc46" style="--depth: 0;"> <a 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against forged sender addresses</span> </a> </li><li class="astro-g2bywc46" style="--depth: 0;"> <a href="#test-it" class="astro-g2bywc46" style="--depth: 0;"> <span class="astro-g2bywc46" style="--depth: 0;">Test it</span> </a> <ul class="astro-g2bywc46" style="--depth: 1;"> <li class="astro-g2bywc46" style="--depth: 1;"> <a href="#no-encryption-no-authentication" class="astro-g2bywc46" style="--depth: 1;"> <span class="astro-g2bywc46" style="--depth: 1;">No encryption, no authentication</span> </a> </li><li class="astro-g2bywc46" style="--depth: 1;"> <a href="#with-encryption-no-authentication" class="astro-g2bywc46" style="--depth: 1;"> <span class="astro-g2bywc46" style="--depth: 1;">With encryption, no authentication</span> </a> </li><li class="astro-g2bywc46" style="--depth: 1;"> <a href="#with-encryption-and-authentication" class="astro-g2bywc46" style="--depth: 1;"> <span class="astro-g2bywc46" style="--depth: 1;">With encryption and authentication</span> </a> </li> </ul> </li><li class="astro-g2bywc46" style="--depth: 0;"> <a href="#gory-details-of-authentication" class="astro-g2bywc46" style="--depth: 0;"> <span class="astro-g2bywc46" style="--depth: 0;">Gory details of authentication</span> </a> </li><li class="astro-g2bywc46" style="--depth: 0;"> <a href="#footnote-label" class="astro-g2bywc46" style="--depth: 0;"> <span class="astro-g2bywc46" style="--depth: 0;">Footnotes</span> </a> </li> </ul> </nav></starlight-toc><script type="module" src="/_astro/TableOfContents.astro_astro_type_script_index_0_lang.CKWWgpjV.js"></script></div></div> </div> </aside> <div class="main-pane astro-67yu43on"> <main data-pagefind-body class="astro-bguv2lll" lang="en" dir="ltr"> <div class="content-panel astro-7nkwcw3z"> <div class="sl-container astro-7nkwcw3z"> <h1 id="_top" class="astro-j6tvhyss">Sending / Relaying</h1> </div> </div> <div class="content-panel astro-7nkwcw3z"> <div class="sl-container astro-7nkwcw3z"> <div class="sl-markdown-content"> <p>Your mail server is almost ready for use. But one puzzle piece is missing. Your users can receive emails but they cannot
send them yet. This part of the guide is a bit longer than others because forwarding emails has security implications.</p>
<p>Lets begin with the difference of how <strong>users</strong> send emails versus how <strong>mail servers</strong> send emails.</p>
<ul>
<li>A <strong>mail server</strong> fetches the <em>MX</em> record for the destination domain of the recipients email address from DNS. That
tells it which mail server to talk to. Then it opens an SMTP connection (TCP port 25) and sends the email. A mail
server will not relay an email meant for someone else. It will only accept emails for its own users.</li>
<li>An <strong>end user</strong> with a mail client such as Thunderbird or Roundcube cannot do that. Mail clients do not include any
functionality for fetching MX records. In addition, the user is usually on a dynamic IP address that other mail
servers will not trust and will likely reject. Instead, end users are expected to send their emails through their
providers (=your) mail server: they authenticate with their login credentials and then send the message. This process
is called relaying, because your mail server acts as an intermediary between the user and other mail servers on the
internet. For security reasons, authentication is required before the user is allowed to send emails. Users are also
expected to use the TCP port 587 which is called the <em>submission</em> port. More on that later.</li>
</ul>
<p>Postfix must only accept an email <strong>from a user</strong>, if…</p>
<ul>
<li>The user uses the <strong>submission</strong> port (and not the standard SMTP port)</li>
<li>The user <strong>authenticates</strong> with the username (in our setup: the email address) and the password</li>
<li>The connection is <strong>encrypted</strong> so that the password can be safely transmitted</li>
<li>The <strong>sender address</strong> of the email matches the users account</li>
</ul>
<div class="sl-heading-wrapper level-h2"><h2 id="smtp-versus-submission">SMTP versus Submission</h2><a class="sl-anchor-link" href="#smtp-versus-submission"><span aria-hidden="true" class="sl-anchor-icon"><svg width="16" height="16" viewBox="0 0 24 24"><path fill="currentcolor" d="m12.11 15.39-3.88 3.88a2.52 2.52 0 0 1-3.5 0 2.47 2.47 0 0 1 0-3.5l3.88-3.88a1 1 0 0 0-1.42-1.42l-3.88 3.89a4.48 4.48 0 0 0 6.33 6.33l3.89-3.88a1 1 0 1 0-1.42-1.42Zm8.58-12.08a4.49 4.49 0 0 0-6.33 0l-3.89 3.88a1 1 0 0 0 1.42 1.42l3.88-3.88a2.52 2.52 0 0 1 3.5 0 2.47 2.47 0 0 1 0 3.5l-3.88 3.88a1 1 0 1 0 1.42 1.42l3.88-3.89a4.49 4.49 0 0 0 0-6.33ZM8.83 15.17a1 1 0 0 0 1.1.22 1 1 0 0 0 .32-.22l4.92-4.92a1 1 0 0 0-1.42-1.42l-4.92 4.92a1 1 0 0 0 0 1.42Z"></path></svg></span><span class="sr-only">Section titled “SMTP versus Submission”</span></a></div>
<p>Lets compare those services and ports side-by-side:</p>
<table><thead><tr><th>Service name</th><th>SMTP</th><th>Submission</th><th>SubmissionS (S=secure)</th></tr></thead><tbody><tr><td>TCP Port</td><td>25</td><td>587</td><td>465</td></tr><tr><td>Meant for</td><td>Other mail servers</td><td>Our users</td><td>Our users</td></tr><tr><td>Used by</td><td>Mail transport agent (MTA)</td><td>Mail user agent (MUA)</td><td>Mail user agent (MUA)</td></tr><tr><td>Transport encryption</td><td><strong>Off</strong>. STARTTLS optional.</td><td><strong>Off</strong>. STARTTLS mandatory.</td><td>TLS all the time</td></tr><tr><td>Authentication</td><td>Optional <sup><a href="#user-content-fn-1" id="user-content-fnref-1" data-footnote-ref="" aria-describedby="footnote-label">1</a></sup></td><td>Mandatory</td><td>Mandatory</td></tr><tr><td>Accepted sender</td><td>Anyone</td><td>Our own users</td><td>Our own users</td></tr><tr><td>Accepted receivers</td><td>Our own users</td><td>Anyone</td><td>Anyone</td></tr><tr><td>Your home ISP</td><td>may block this port</td><td>will allow this port</td><td>will allow this port</td></tr></tbody></table>
<p>I hope this helps clarify the distinction. Human users must use the submission service on either port 587 or 465. The
key difference is that connections on port 587 start unencrypted, and the email client must issue the STARTTLS command
to enable encryption. In contrast, port 465 requires the email client to use TLS encryption right from the first byte of
communication.</p>
<div class="sl-heading-wrapper level-h2"><h2 id="configure-the-submissions-services">Configure the submission[s] services</h2><a class="sl-anchor-link" href="#configure-the-submissions-services"><span aria-hidden="true" class="sl-anchor-icon"><svg width="16" height="16" viewBox="0 0 24 24"><path fill="currentcolor" d="m12.11 15.39-3.88 3.88a2.52 2.52 0 0 1-3.5 0 2.47 2.47 0 0 1 0-3.5l3.88-3.88a1 1 0 0 0-1.42-1.42l-3.88 3.89a4.48 4.48 0 0 0 6.33 6.33l3.89-3.88a1 1 0 1 0-1.42-1.42Zm8.58-12.08a4.49 4.49 0 0 0-6.33 0l-3.89 3.88a1 1 0 0 0 1.42 1.42l3.88-3.88a2.52 2.52 0 0 1 3.5 0 2.47 2.47 0 0 1 0 3.5l-3.88 3.88a1 1 0 1 0 1.42 1.42l3.88-3.89a4.49 4.49 0 0 0 0-6.33ZM8.83 15.17a1 1 0 0 0 1.1.22 1 1 0 0 0 .32-.22l4.92-4.92a1 1 0 0 0-1.42-1.42l-4.92 4.92a1 1 0 0 0 0 1.42Z"></path></svg></span><span class="sr-only">Section titled “Configure the submission[s] services”</span></a></div>
<p>All Postfix services are declared in the <code dir="auto">/etc/postfix/master.cf</code> file. Most of them are used by Postfix internally,
like <code dir="auto">pickup</code>, <code dir="auto">cleanup</code>, <code dir="auto">rewrite</code> or <code dir="auto">bounce</code>. But <code dir="auto">submission</code> is not configured by default. Lets fix that.</p>
<div class="expressive-code"><link rel="stylesheet" href="/_astro/ec.v4551.css"><script type="module" src="/_astro/ec.0vx5m.js"></script><figure class="frame is-terminal has-title not-content"><figcaption class="header"><span class="title">Run this on your server</span></figcaption><pre data-language="sh"><code><div class="ec-line"><div class="code"><span style="--0:#82AAFF;--1:#3B61B0">postconf</span><span style="--0:#D6DEEB;--1:#403F53"> </span><span style="--0:#82AAFF;--1:#3B61B0">-M</span><span style="--0:#D6DEEB;--1:#403F53"> </span><span style="--0:#ECC48D;--1:#3B61B0">submission/inet=</span><span style="--0:#D9F5DD;--1:#111111">"</span><span style="--0:#ECC48D;--1:#984E4D">submission inet n - y - - smtpd</span><span style="--0:#D9F5DD;--1:#111111">"</span></div></div><div class="ec-line"><div class="code"><span style="--0:#82AAFF;--1:#3B61B0">postconf</span><span style="--0:#D6DEEB;--1:#403F53"> </span><span style="--0:#82AAFF;--1:#3B61B0">-P</span><span style="--0:#D6DEEB;--1:#403F53"> </span><span style="--0:#D9F5DD;--1:#111111">"</span><span style="--0:#ECC48D;--1:#984E4D">submission/inet/syslog_name=postfix/submission</span><span style="--0:#D9F5DD;--1:#111111">"</span></div></div><div class="ec-line"><div class="code"><span style="--0:#82AAFF;--1:#3B61B0">postconf</span><span style="--0:#D6DEEB;--1:#403F53"> </span><span style="--0:#82AAFF;--1:#3B61B0">-P</span><span style="--0:#D6DEEB;--1:#403F53"> </span><span style="--0:#D9F5DD;--1:#111111">"</span><span style="--0:#ECC48D;--1:#984E4D">submission/inet/smtpd_tls_security_level=encrypt</span><span style="--0:#D9F5DD;--1:#111111">"</span></div></div><div class="ec-line"><div class="code"><span style="--0:#82AAFF;--1:#3B61B0">postconf</span><span style="--0:#D6DEEB;--1:#403F53"> </span><span style="--0:#82AAFF;--1:#3B61B0">-P</span><span style="--0:#D6DEEB;--1:#403F53"> </span><span style="--0:#D9F5DD;--1:#111111">"</span><span style="--0:#ECC48D;--1:#984E4D">submission/inet/smtpd_sasl_auth_enable=yes</span><span style="--0:#D9F5DD;--1:#111111">"</span></div></div><div class="ec-line"><div class="code"><span style="--0:#82AAFF;--1:#3B61B0">postconf</span><span style="--0:#D6DEEB;--1:#403F53"> </span><span style="--0:#82AAFF;--1:#3B61B0">-P</span><span style="--0:#D6DEEB;--1:#403F53"> </span><span style="--0:#D9F5DD;--1:#111111">"</span><span style="--0:#ECC48D;--1:#984E4D">submission/inet/smtpd_sasl_type=dovecot</span><span style="--0:#D9F5DD;--1:#111111">"</span></div></div><div class="ec-line"><div class="code"><span style="--0:#82AAFF;--1:#3B61B0">postconf</span><span style="--0:#D6DEEB;--1:#403F53"> </span><span style="--0:#82AAFF;--1:#3B61B0">-P</span><span style="--0:#D6DEEB;--1:#403F53"> </span><span style="--0:#D9F5DD;--1:#111111">"</span><span style="--0:#ECC48D;--1:#984E4D">submission/inet/smtpd_sasl_path=private/dovecot-auth</span><span style="--0:#D9F5DD;--1:#111111">"</span></div></div><div class="ec-line"><div class="code"><span style="--0:#82AAFF;--1:#3B61B0">postconf</span><span style="--0:#D6DEEB;--1:#403F53"> </span><span style="--0:#82AAFF;--1:#3B61B0">-P</span><span style="--0:#D6DEEB;--1:#403F53"> </span><span style="--0:#D9F5DD;--1:#111111">"</span><span style="--0:#ECC48D;--1:#984E4D">submission/inet/smtpd_recipient_restrictions=permit_sasl_authenticated,reject</span><span style="--0:#D9F5DD;--1:#111111">"</span></div></div><div class="ec-line"><div class="code"><span style="--0:#82AAFF;--1:#3B61B0">postconf</span><span style="--0:#D6DEEB;--1:#403F53"> </span><span style="--0:#82AAFF;--1:#3B61B0">-P</span><span style="--0:#D6DEEB;--1:#403F53"> </span><span style="--0:#D9F5DD;--1:#111111">"</span><span style="--0:#ECC48D;--1:#984E4D">submission/inet/smtpd_sender_restrictions=reject_sender_login_mismatch,permit_sasl_authenticated,reject</span><span style="--0:#D9F5DD;--1:#111111">"</span></div></div><div class="ec-line"><div class="code">
</div></div><div class="ec-line"><div class="code"><span style="--0:#82AAFF;--1:#3B61B0">postfix</span><span style="--0:#D6DEEB;--1:#403F53"> </span><span style="--0:#ECC48D;--1:#3B61B0">reload</span></div></div></code></pre><div class="copy"><div aria-live="polite"></div><button title="Copy to clipboard" data-copied="Copied!" data-code="postconf -M submission/inet=&#x22;submission inet n - y - - smtpd&#x22;postconf -P &#x22;submission/inet/syslog_name=postfix/submission&#x22;postconf -P &#x22;submission/inet/smtpd_tls_security_level=encrypt&#x22;postconf -P &#x22;submission/inet/smtpd_sasl_auth_enable=yes&#x22;postconf -P &#x22;submission/inet/smtpd_sasl_type=dovecot&#x22;postconf -P &#x22;submission/inet/smtpd_sasl_path=private/dovecot-auth&#x22;postconf -P &#x22;submission/inet/smtpd_recipient_restrictions=permit_sasl_authenticated,reject&#x22;postconf -P &#x22;submission/inet/smtpd_sender_restrictions=reject_sender_login_mismatch,permit_sasl_authenticated,reject&#x22;postfix reload"><div></div></button></div></figure></div>
<p>These commands added a <code dir="auto">submission</code> service to the <code dir="auto">/etc/postfix/master.cf</code> file. You could have achieved the same thing
by editing that file and adding the following section, but I wanted to make it simpler for you to just copy/paste the
commands into your shell.</p>
<div class="expressive-code"><figure class="frame has-title not-content"><figcaption class="header"><span class="title">What it did…</span></figcaption><pre data-language="text"><code><div class="ec-line"><div class="code"><span style="--0:#d6deeb;--1:#403f53">submission inet n - y - - smtpd</span></div></div><div class="ec-line"><div class="code"><span class="indent"><span style="--0:#d6deeb;--1:#403f53"> </span></span><span style="--0:#d6deeb;--1:#403f53">-o syslog_name=postfix/submission</span></div></div><div class="ec-line"><div class="code"><span class="indent"><span style="--0:#d6deeb;--1:#403f53"> </span></span><span style="--0:#d6deeb;--1:#403f53">-o smtpd_tls_security_level=encrypt</span></div></div><div class="ec-line"><div class="code"><span class="indent"><span style="--0:#d6deeb;--1:#403f53"> </span></span><span style="--0:#d6deeb;--1:#403f53">-o smtpd_sasl_auth_enable=yes</span></div></div><div class="ec-line"><div class="code"><span class="indent"><span style="--0:#d6deeb;--1:#403f53"> </span></span><span style="--0:#d6deeb;--1:#403f53">-o smtpd_sasl_type=dovecot</span></div></div><div class="ec-line"><div class="code"><span class="indent"><span style="--0:#d6deeb;--1:#403f53"> </span></span><span style="--0:#d6deeb;--1:#403f53">-o smtpd_sasl_path=private/dovecot-auth</span></div></div><div class="ec-line"><div class="code"><span class="indent"><span style="--0:#d6deeb;--1:#403f53"> </span></span><span style="--0:#d6deeb;--1:#403f53">-o smtpd_recipient_restrictions=permit_sasl_authenticated,reject</span></div></div></code></pre><div class="copy"><div aria-live="polite"></div><button title="Copy to clipboard" data-copied="Copied!" data-code="submission inet n - y - - smtpd -o syslog_name=postfix/submission -o smtpd_tls_security_level=encrypt -o smtpd_sasl_auth_enable=yes -o smtpd_sasl_type=dovecot -o smtpd_sasl_path=private/dovecot-auth -o smtpd_recipient_restrictions=permit_sasl_authenticated,reject"><div></div></button></div></figure></div>
<p>Some notes on that:</p>
<ul>
<li>Lines with <strong>options</strong> (<code dir="auto">-o</code>) need to be indented so that Postfix understands that they belong to the same service.</li>
<li>The word <strong>submission</strong> refers to the definition of the service in the <code dir="auto">/etc/services</code> file. So it translates to TCP
port 587.</li>
<li>The <strong>smtpd</strong> in the last column of the first row tells Postfix that connections should be handled as incoming SMTP
connections.</li>
<li><strong>syslog_name</strong> is adding <code dir="auto">postfix/submission</code> to log files so that you can distinguish it from other connections.</li>
<li><strong>smtpd_tls_security_level</strong> enabled and enforces the use of STARTTLS. The connection starts without encryption but
the remote party is required to send the <code dir="auto">STARTTLS</code> command to switch to an encrypted connection.</li>
<li><strong>smtpd_sasl_auth_enable</strong> enables authentication on this port. SASL is short for <em>Simple Authentication and Security
Layer</em>. That way only our own users can use this service.</li>
<li><strong>smtpd_sasl_type</strong> tells Postfix to use Dovecots service to authentication a uaser.</li>
<li><strong>smtpd_sasl_path</strong> tells Postfix that it should talk to Dovecot through the socket file
<code dir="auto">/var/spool/postfix/private/dovecot-auth</code>. Earlier in this guide you told Dovecot to create that socket at that
location.</li>
<li><strong>smtpd_recipient_restrictions</strong> define that only authenticated connections are permitted. Without a username and a
password your connection will be rejected. The <strong>recipient</strong> keyword tells us that this restricts who the recipient
can be.</li>
<li><strong>smtpd_sender_restrictions</strong> restricts who the <strong>sender</strong> may be. I will explain that
<a href="#protecting-against-forged-sender-addresses">later</a>.</li>
</ul>
<p>While you are at it, create the <code dir="auto">submissions</code> (with an “s” at the end) service as well:</p>
<div class="expressive-code"><figure class="frame is-terminal has-title not-content"><figcaption class="header"><span class="title">Run this on your server</span></figcaption><pre data-language="sh"><code><div class="ec-line"><div class="code"><span style="--0:#82AAFF;--1:#3B61B0">postconf</span><span style="--0:#D6DEEB;--1:#403F53"> </span><span style="--0:#82AAFF;--1:#3B61B0">-M</span><span style="--0:#D6DEEB;--1:#403F53"> </span><span style="--0:#ECC48D;--1:#3B61B0">submissions/inet=</span><span style="--0:#D9F5DD;--1:#111111">"</span><span style="--0:#ECC48D;--1:#984E4D">submissions inet n - y - - smtpd</span><span style="--0:#D9F5DD;--1:#111111">"</span></div></div><div class="ec-line"><div class="code"><span style="--0:#82AAFF;--1:#3B61B0">postconf</span><span style="--0:#D6DEEB;--1:#403F53"> </span><span style="--0:#82AAFF;--1:#3B61B0">-P</span><span style="--0:#D6DEEB;--1:#403F53"> </span><span style="--0:#D9F5DD;--1:#111111">"</span><span style="--0:#ECC48D;--1:#984E4D">submissions/inet/syslog_name=postfix/submissions</span><span style="--0:#D9F5DD;--1:#111111">"</span></div></div><div class="ec-line"><div class="code"><span style="--0:#82AAFF;--1:#3B61B0">postconf</span><span style="--0:#D6DEEB;--1:#403F53"> </span><span style="--0:#82AAFF;--1:#3B61B0">-P</span><span style="--0:#D6DEEB;--1:#403F53"> </span><span style="--0:#D9F5DD;--1:#111111">"</span><span style="--0:#ECC48D;--1:#984E4D">submissions/inet/smtpd_tls_wrappermode=yes</span><span style="--0:#D9F5DD;--1:#111111">"</span></div></div><div class="ec-line"><div class="code"><span style="--0:#82AAFF;--1:#3B61B0">postconf</span><span style="--0:#D6DEEB;--1:#403F53"> </span><span style="--0:#82AAFF;--1:#3B61B0">-P</span><span style="--0:#D6DEEB;--1:#403F53"> </span><span style="--0:#D9F5DD;--1:#111111">"</span><span style="--0:#ECC48D;--1:#984E4D">submissions/inet/smtpd_sasl_auth_enable=yes</span><span style="--0:#D9F5DD;--1:#111111">"</span></div></div><div class="ec-line"><div class="code"><span style="--0:#82AAFF;--1:#3B61B0">postconf</span><span style="--0:#D6DEEB;--1:#403F53"> </span><span style="--0:#82AAFF;--1:#3B61B0">-P</span><span style="--0:#D6DEEB;--1:#403F53"> </span><span style="--0:#D9F5DD;--1:#111111">"</span><span style="--0:#ECC48D;--1:#984E4D">submissions/inet/smtpd_sasl_type=dovecot</span><span style="--0:#D9F5DD;--1:#111111">"</span></div></div><div class="ec-line"><div class="code"><span style="--0:#82AAFF;--1:#3B61B0">postconf</span><span style="--0:#D6DEEB;--1:#403F53"> </span><span style="--0:#82AAFF;--1:#3B61B0">-P</span><span style="--0:#D6DEEB;--1:#403F53"> </span><span style="--0:#D9F5DD;--1:#111111">"</span><span style="--0:#ECC48D;--1:#984E4D">submissions/inet/smtpd_sasl_path=private/dovecot-auth</span><span style="--0:#D9F5DD;--1:#111111">"</span></div></div><div class="ec-line"><div class="code"><span style="--0:#82AAFF;--1:#3B61B0">postconf</span><span style="--0:#D6DEEB;--1:#403F53"> </span><span style="--0:#82AAFF;--1:#3B61B0">-P</span><span style="--0:#D6DEEB;--1:#403F53"> </span><span style="--0:#D9F5DD;--1:#111111">"</span><span style="--0:#ECC48D;--1:#984E4D">submissions/inet/smtpd_recipient_restrictions=permit_sasl_authenticated,reject</span><span style="--0:#D9F5DD;--1:#111111">"</span></div></div><div class="ec-line"><div class="code"><span style="--0:#82AAFF;--1:#3B61B0">postconf</span><span style="--0:#D6DEEB;--1:#403F53"> </span><span style="--0:#82AAFF;--1:#3B61B0">-P</span><span style="--0:#D6DEEB;--1:#403F53"> </span><span style="--0:#D9F5DD;--1:#111111">"</span><span style="--0:#ECC48D;--1:#984E4D">submissions/inet/smtpd_sender_restrictions=reject_sender_login_mismatch,permit_sasl_authenticated,reject</span><span style="--0:#D9F5DD;--1:#111111">"</span></div></div><div class="ec-line"><div class="code"><span style="--0:#82AAFF;--1:#3B61B0">postfix</span><span style="--0:#D6DEEB;--1:#403F53"> </span><span style="--0:#ECC48D;--1:#3B61B0">reload</span></div></div></code></pre><div class="copy"><div aria-live="polite"></div><button title="Copy to clipboard" data-copied="Copied!" data-code="postconf -M submissions/inet=&#x22;submissions inet n - y - - smtpd&#x22;postconf -P &#x22;submissions/inet/syslog_name=postfix/submissions&#x22;postconf -P &#x22;submissions/inet/smtpd_tls_wrappermode=yes&#x22;postconf -P &#x22;submissions/inet/smtpd_sasl_auth_enable=yes&#x22;postconf -P &#x22;submissions/inet/smtpd_sasl_type=dovecot&#x22;postconf -P &#x22;submissions/inet/smtpd_sasl_path=private/dovecot-auth&#x22;postconf -P &#x22;submissions/inet/smtpd_recipient_restrictions=permit_sasl_authenticated,reject&#x22;postconf -P &#x22;submissions/inet/smtpd_sender_restrictions=reject_sender_login_mismatch,permit_sasl_authenticated,reject&#x22;postfix reload"><div></div></button></div></figure></div>
<p>The only difference to the <code dir="auto">submission</code> (without the “s” at the end) is the <code dir="auto">smtpd_tls_wrappermode</code> that switches on TLS
encryption right from the start of a connection.</p>
<div class="sl-heading-wrapper level-h2"><h2 id="set-up-encryption">Set up encryption</h2><a class="sl-anchor-link" href="#set-up-encryption"><span aria-hidden="true" class="sl-anchor-icon"><svg width="16" height="16" viewBox="0 0 24 24"><path fill="currentcolor" d="m12.11 15.39-3.88 3.88a2.52 2.52 0 0 1-3.5 0 2.47 2.47 0 0 1 0-3.5l3.88-3.88a1 1 0 0 0-1.42-1.42l-3.88 3.89a4.48 4.48 0 0 0 6.33 6.33l3.89-3.88a1 1 0 1 0-1.42-1.42Zm8.58-12.08a4.49 4.49 0 0 0-6.33 0l-3.89 3.88a1 1 0 0 0 1.42 1.42l3.88-3.88a2.52 2.52 0 0 1 3.5 0 2.47 2.47 0 0 1 0 3.5l-3.88 3.88a1 1 0 1 0 1.42 1.42l3.88-3.89a4.49 4.49 0 0 0 0-6.33ZM8.83 15.17a1 1 0 0 0 1.1.22 1 1 0 0 0 .32-.22l4.92-4.92a1 1 0 0 0-1.42-1.42l-4.92 4.92a1 1 0 0 0 0 1.42Z"></path></svg></span><span class="sr-only">Section titled “Set up encryption”</span></a></div>
<p>Whenever possible we should encrypt our traffic. For our users passwords we make it mandatory. But even for
server-to-server connections we should encourage it. Do not confuse it with end-to-end encryption of emails. That
required using PGP or S/MIME. What we are dealing here is <em>transport encryption</em>. It encrypts the communication between
a peer server and your mail server. What the peer server does with our email is beyond our control.</p>
<aside aria-label="Important" class="starlight-aside starlight-aside--danger"> <p class="starlight-aside__title" aria-hidden="true"> <svg aria-hidden="true" class="starlight-aside__icon astro-c6vsoqas" width="16" height="16" viewBox="0 0 24 24" fill="currentColor" style="--sl-icon-size: 1em;"><path d="M12 7a1 1 0 0 0-1 1v4a1 1 0 0 0 2 0V8a1 1 0 0 0-1-1Zm0 8a1 1 0 1 0 0 2 1 1 0 0 0 0-2Zm9.71-7.44-5.27-5.27a1.05 1.05 0 0 0-.71-.29H8.27a1.05 1.05 0 0 0-.71.29L2.29 7.56a1.05 1.05 0 0 0-.29.71v7.46c.004.265.107.518.29.71l5.27 5.27c.192.183.445.286.71.29h7.46a1.05 1.05 0 0 0 .71-.29l5.27-5.27a1.05 1.05 0 0 0 .29-.71V8.27a1.05 1.05 0 0 0-.29-.71ZM20 15.31 15.31 20H8.69L4 15.31V8.69L8.69 4h6.62L20 8.69v6.62Z"/></svg>Important </p> <div class="starlight-aside__content"> <p>Please replace <code dir="auto">mail.example.org</code> by the FQDN you chose.</p> </div> </aside>
<div class="expressive-code"><figure class="frame is-terminal has-title not-content"><figcaption class="header"><span class="title">Run this on your server</span></figcaption><pre data-language="sh"><code><div class="ec-line"><div class="code"><span style="--0:#82AAFF;--1:#3B61B0">postconf</span><span style="--0:#D6DEEB;--1:#403F53"> </span><span style="--0:#ECC48D;--1:#3B61B0">smtp_tls_security_level=encrypt</span></div></div><div class="ec-line"><div class="code"><span style="--0:#82AAFF;--1:#3B61B0">postconf</span><span style="--0:#D6DEEB;--1:#403F53"> </span><span style="--0:#ECC48D;--1:#3B61B0">smtpd_tls_security_level=encrypt</span></div></div><div class="ec-line"><div class="code"><span style="--0:#82AAFF;--1:#3B61B0">postconf</span><span style="--0:#D6DEEB;--1:#403F53"> </span><span style="--0:#ECC48D;--1:#3B61B0">smtp_tls_mandatory_protocols=</span><span style="--0:#D9F5DD;--1:#111111">"</span><span style="--0:#ECC48D;--1:#984E4D">>=TLSv1.2</span><span style="--0:#D9F5DD;--1:#111111">"</span></div></div><div class="ec-line"><div class="code"><span style="--0:#82AAFF;--1:#3B61B0">postconf</span><span style="--0:#D6DEEB;--1:#403F53"> </span><span style="--0:#ECC48D;--1:#3B61B0">smtpd_tls_mandatory_protocols=</span><span style="--0:#D9F5DD;--1:#111111">"</span><span style="--0:#ECC48D;--1:#984E4D">>=TLSv1.2</span><span style="--0:#D9F5DD;--1:#111111">"</span></div></div><div class="ec-line"><div class="code"><span style="--0:#82AAFF;--1:#3B61B0">postconf</span><span style="--0:#D6DEEB;--1:#403F53"> </span><span style="--0:#ECC48D;--1:#3B61B0">smtp_tls_mandatory_ciphers=high</span></div></div><div class="ec-line"><div class="code"><span style="--0:#82AAFF;--1:#3B61B0">postconf</span><span style="--0:#D6DEEB;--1:#403F53"> </span><span style="--0:#ECC48D;--1:#3B61B0">smtpd_tls_mandatory_ciphers=high</span></div></div><div class="ec-line"><div class="code"><span style="--0:#82AAFF;--1:#3B61B0">postconf</span><span style="--0:#D6DEEB;--1:#403F53"> </span><span style="--0:#ECC48D;--1:#3B61B0">smtpd_tls_cert_file=/etc/letsencrypt/live/mail.example.org/fullchain.pem</span></div></div><div class="ec-line"><div class="code"><span style="--0:#82AAFF;--1:#3B61B0">postconf</span><span style="--0:#D6DEEB;--1:#403F53"> </span><span style="--0:#ECC48D;--1:#3B61B0">smtpd_tls_key_file=/etc/letsencrypt/live/mail.example.org/privkey.pem</span></div></div></code></pre><div class="copy"><div aria-live="polite"></div><button title="Copy to clipboard" data-copied="Copied!" data-code="postconf smtp_tls_security_level=encryptpostconf smtpd_tls_security_level=encryptpostconf smtp_tls_mandatory_protocols=&#x22;>=TLSv1.2&#x22;postconf smtpd_tls_mandatory_protocols=&#x22;>=TLSv1.2&#x22;postconf smtp_tls_mandatory_ciphers=highpostconf smtpd_tls_mandatory_ciphers=highpostconf smtpd_tls_cert_file=/etc/letsencrypt/live/mail.example.org/fullchain.pempostconf smtpd_tls_key_file=/etc/letsencrypt/live/mail.example.org/privkey.pem"><div></div></button></div></figure></div>
<p>What it means:</p>
<ul>
<li><strong>smtp_tls_security_level</strong>: <br>
Enforce encrypted <em>outgoing</em> SMTP connections.</li>
<li><strong>smtpd_tls_security_level</strong>: <br>
Enforce encrypted <em>incoming</em> SMTP connections.</li>
<li><strong>smtp_tls_mandatory_protocols</strong> / <strong>smtpd_tls_mandatory_protocols</strong>: <br>
Only allow recommended versions of <a href="https://en.wikipedia.org/wiki/Transport_Layer_Security">TLS</a>. Version 1.2 has been
introduced 2008. Version 1.3 is from 2018.</li>
<li><strong>smtp_tls_mandatory_ciphers</strong> / <strong>smtpd_tls_mandatory_ciphers</strong>: <br>
The list of encryption algorithms that you allow for communnication. Default is <em>medium</em>.</li>
<li><strong>smtpd_tls_cert_file</strong> and <strong>smtpd_tls_key_file</strong>: <br>
Where to find the private key and certificate for encryption.</li>
</ul>
<aside aria-label="Losen the encryption?" class="starlight-aside starlight-aside--tip"> <p class="starlight-aside__title" aria-hidden="true"> <svg aria-hidden="true" class="starlight-aside__icon astro-c6vsoqas" width="16" height="16" viewBox="0 0 24 24" fill="currentColor" style="--sl-icon-size: 1em;"><path fill-rule="evenodd" d="M1.44 8.855v-.001l3.527-3.516c.34-.344.802-.541 1.285-.548h6.649l.947-.947c3.07-3.07 6.207-3.072 7.62-2.868a1.821 1.821 0 0 1 1.557 1.557c.204 1.413.203 4.55-2.868 7.62l-.946.946v6.649a1.845 1.845 0 0 1-.549 1.286l-3.516 3.528a1.844 1.844 0 0 1-3.11-.944l-.858-4.275-4.52-4.52-2.31-.463-1.964-.394A1.847 1.847 0 0 1 .98 10.693a1.843 1.843 0 0 1 .46-1.838Zm5.379 2.017-3.873-.776L6.32 6.733h4.638l-4.14 4.14Zm8.403-5.655c2.459-2.46 4.856-2.463 5.89-2.33.134 1.035.13 3.432-2.329 5.891l-6.71 6.71-3.561-3.56 6.71-6.711Zm-1.318 15.837-.776-3.873 4.14-4.14v4.639l-3.364 3.374Z" clip-rule="evenodd"/><path d="M9.318 18.345a.972.972 0 0 0-1.86-.561c-.482 1.435-1.687 2.204-2.934 2.619a8.22 8.22 0 0 1-1.23.302c.062-.365.157-.79.303-1.229.415-1.247 1.184-2.452 2.62-2.935a.971.971 0 1 0-.62-1.842c-.12.04-.236.084-.35.13-2.02.828-3.012 2.588-3.493 4.033a10.383 10.383 0 0 0-.51 2.845l-.001.016v.063c0 .536.434.972.97.972H2.24a7.21 7.21 0 0 0 .878-.065c.527-.063 1.248-.19 2.02-.447 1.445-.48 3.205-1.472 4.033-3.494a5.828 5.828 0 0 0 .147-.407Z"/></svg>Losen the encryption? </p> <div class="starlight-aside__content"> <p>In rare cases you may fail to communicate with other computers because they only support older TLS versions (up to
1.1) or ciphers that are not considered secure nowadays. In that case you can remove the <code dir="auto">smtpd_tls_security_level</code>,
<code dir="auto">smtpd_tls_mandatory_protocols</code> and <code dir="auto">smtpd_tls_mandatory_ciphers</code> settings from your <code dir="auto">main.cf</code> file and fall back to
the relaxed security requirements.</p> </div> </aside>
<p>Please note that configuration in <code dir="auto">main.cf</code> applies to all Postfix services. So the above configuration enabled
encryption for both server-to-server and human-to-server (<code dir="auto">submission</code>) services. The <code dir="auto">smtpd_sasl_auth_enable</code> will only
be set for the <code dir="auto">submission</code> service to enable authentication there.<sup><a href="#user-content-fn-1" id="user-content-fnref-1-2" data-footnote-ref="" aria-describedby="footnote-label">1</a></sup></p>
<aside aria-label="smtp or smtpd?" class="starlight-aside starlight-aside--tip"> <p class="starlight-aside__title" aria-hidden="true"> <svg aria-hidden="true" class="starlight-aside__icon astro-c6vsoqas" width="16" height="16" viewBox="0 0 24 24" fill="currentColor" style="--sl-icon-size: 1em;"><path fill-rule="evenodd" d="M1.44 8.855v-.001l3.527-3.516c.34-.344.802-.541 1.285-.548h6.649l.947-.947c3.07-3.07 6.207-3.072 7.62-2.868a1.821 1.821 0 0 1 1.557 1.557c.204 1.413.203 4.55-2.868 7.62l-.946.946v6.649a1.845 1.845 0 0 1-.549 1.286l-3.516 3.528a1.844 1.844 0 0 1-3.11-.944l-.858-4.275-4.52-4.52-2.31-.463-1.964-.394A1.847 1.847 0 0 1 .98 10.693a1.843 1.843 0 0 1 .46-1.838Zm5.379 2.017-3.873-.776L6.32 6.733h4.638l-4.14 4.14Zm8.403-5.655c2.459-2.46 4.856-2.463 5.89-2.33.134 1.035.13 3.432-2.329 5.891l-6.71 6.71-3.561-3.56 6.71-6.711Zm-1.318 15.837-.776-3.873 4.14-4.14v4.639l-3.364 3.374Z" clip-rule="evenodd"/><path d="M9.318 18.345a.972.972 0 0 0-1.86-.561c-.482 1.435-1.687 2.204-2.934 2.619a8.22 8.22 0 0 1-1.23.302c.062-.365.157-.79.303-1.229.415-1.247 1.184-2.452 2.62-2.935a.971.971 0 1 0-.62-1.842c-.12.04-.236.084-.35.13-2.02.828-3.012 2.588-3.493 4.033a10.383 10.383 0 0 0-.51 2.845l-.001.016v.063c0 .536.434.972.97.972H2.24a7.21 7.21 0 0 0 .878-.065c.527-.063 1.248-.19 2.02-.447 1.445-.48 3.205-1.472 4.033-3.494a5.828 5.828 0 0 0 .147-.407Z"/></svg>smtp or smtpd? </p> <div class="starlight-aside__content"> <p>Look closely. Some settings start with <code dir="auto">smtp_</code> and others start with with <code dir="auto">smtpd_</code>. That is not a typo. <code dir="auto">smtp_</code> refers
to the SMTP <em>client</em>. That is the component that sends out emails <strong>from</strong> Postfix <strong>to</strong> other servers. Whereas
<code dir="auto">smtpd_</code> means the SMTP <em>server.</em> That in turn is the component that <strong>receives</strong> emails <strong>from</strong> other systems
either from a remote mail server or one of your users.</p> </div> </aside>
<div class="sl-heading-wrapper level-h2"><h2 id="protecting-against-forged-sender-addresses">Protecting against forged sender addresses</h2><a class="sl-anchor-link" href="#protecting-against-forged-sender-addresses"><span aria-hidden="true" class="sl-anchor-icon"><svg width="16" height="16" viewBox="0 0 24 24"><path fill="currentcolor" d="m12.11 15.39-3.88 3.88a2.52 2.52 0 0 1-3.5 0 2.47 2.47 0 0 1 0-3.5l3.88-3.88a1 1 0 0 0-1.42-1.42l-3.88 3.89a4.48 4.48 0 0 0 6.33 6.33l3.89-3.88a1 1 0 1 0-1.42-1.42Zm8.58-12.08a4.49 4.49 0 0 0-6.33 0l-3.89 3.88a1 1 0 0 0 1.42 1.42l3.88-3.88a2.52 2.52 0 0 1 3.5 0 2.47 2.47 0 0 1 0 3.5l-3.88 3.88a1 1 0 1 0 1.42 1.42l3.88-3.89a4.49 4.49 0 0 0 0-6.33ZM8.83 15.17a1 1 0 0 0 1.1.22 1 1 0 0 0 .32-.22l4.92-4.92a1 1 0 0 0-1.42-1.42l-4.92 4.92a1 1 0 0 0 0 1.42Z"></path></svg></span><span class="sr-only">Section titled “Protecting against forged sender addresses”</span></a></div>
<p>Before we can actually test the setup, there is another security issue we need to address. While Postfix is configured
to relay emails only after a successful login with a valid username and password, it does not prevent an authenticated
user from impersonating another sender. For example, someone could log in as <code dir="auto">john@example.org</code> but send an email
appearing to come from <code dir="auto">brunhilde@example.net</code>.</p>
<p>We should fix that. For that purpose Postfix provides a setting called
<a href="http://www.postfix.org/postconf.5.html#smtpd_sender_login_maps">smtpd_sender_login_maps</a>. From the “maps” part you can
deduce that it expects a <em>mapping</em> again:</p>
<ul>
<li><em>Question</em>: who you <strong>claim</strong> to be (emails sender address)</li>
<li><em>Answer</em>: who you <strong>logged in as</strong> (user name after authentiation)</li>
</ul>
<p>If you think of it, the mapping is simple. The <em>question</em> must match the <em>answer</em>. If <code dir="auto">john@example.org</code> wants to be the
sender, he must also authenticate as <code dir="auto">john@example.org</code>.</p>
<p>As we use the email address also as a user name we simply need a mapping where the email address is listed on both
sides. Lets create it:</p>
<div class="expressive-code"><figure class="frame is-terminal has-title not-content"><figcaption class="header"><span class="title">Run this on your server</span></figcaption><pre data-language="sh"><code><div class="ec-line"><div class="code"><span style="--0:#919F9F;--1:#5F636F"># Create a mapping configuration from the user to themself</span></div></div><div class="ec-line"><div class="code"><span style="--0:#82AAFF;--1:#3B61B0">cat</span><span style="--0:#D6DEEB;--1:#403F53"> </span><span style="--0:#7FDBCA;--1:#096E72">></span><span style="--0:#D6DEEB;--1:#403F53"> </span><span style="--0:#ECC48D;--1:#3B61B0">/etc/postfix/mariadb-email2email.cf</span><span style="--0:#D6DEEB;--1:#403F53"> </span><span style="--0:#7FDBCA;--1:#096E72">&#x3C;&#x3C;</span><span style="--0:#D6DEEB;--1:#403F53"> </span><span style="--0:#D9F5DD;--1:#111111">EOF</span></div></div><div class="ec-line"><div class="code"><span style="--0:#ECC48D;--1:#3B61B0">user = mailserver</span></div></div><div class="ec-line"><div class="code"><span style="--0:#ECC48D;--1:#3B61B0">password = MAILSERVER-PASSWORD-HERE</span></div></div><div class="ec-line"><div class="code"><span style="--0:#ECC48D;--1:#3B61B0">hosts = 127.0.0.1</span></div></div><div class="ec-line"><div class="code"><span style="--0:#ECC48D;--1:#3B61B0">dbname = mailserver</span></div></div><div class="ec-line"><div class="code"><span style="--0:#ECC48D;--1:#3B61B0">query = SELECT email FROM virtual_users WHERE email='%s'</span></div></div><div class="ec-line"><div class="code"><span style="--0:#D9F5DD;--1:#111111">EOF</span></div></div><div class="ec-line"><div class="code">
</div></div><div class="ec-line"><div class="code"><span style="--0:#919F9F;--1:#5F636F"># Fix permissions</span></div></div><div class="ec-line"><div class="code"><span style="--0:#82AAFF;--1:#3B61B0">chown</span><span style="--0:#D6DEEB;--1:#403F53"> </span><span style="--0:#ECC48D;--1:#3B61B0">root:postfix</span><span style="--0:#D6DEEB;--1:#403F53"> </span><span style="--0:#ECC48D;--1:#3B61B0">/etc/postfix/mariadb-email2email.cf</span></div></div><div class="ec-line"><div class="code"><span style="--0:#82AAFF;--1:#3B61B0">chmod</span><span style="--0:#D6DEEB;--1:#403F53"> </span><span style="--0:#ECC48D;--1:#3B61B0">u=rw,g=r,o=</span><span style="--0:#D6DEEB;--1:#403F53"> </span><span style="--0:#ECC48D;--1:#3B61B0">/etc/postfix/mariadb-email2email.cf</span></div></div><div class="ec-line"><div class="code">
</div></div><div class="ec-line"><div class="code"><span style="--0:#919F9F;--1:#5F636F"># Tell Postfix to use this mapping</span></div></div><div class="ec-line"><div class="code"><span style="--0:#82AAFF;--1:#3B61B0">postconf</span><span style="--0:#D6DEEB;--1:#403F53"> </span><span style="--0:#ECC48D;--1:#3B61B0">smtpd_sender_login_maps=mysql:/etc/postfix/mariadb-email2email.cf</span></div></div></code></pre><div class="copy"><div aria-live="polite"></div><button title="Copy to clipboard" data-copied="Copied!" data-code="# Create a mapping configuration from the user to themselfcat > /etc/postfix/mariadb-email2email.cf << EOFuser = mailserverpassword = MAILSERVER-PASSWORD-HEREhosts = 127.0.0.1dbname = mailserverquery = SELECT email FROM virtual_users WHERE email=&#x27;%s&#x27;EOF# Fix permissionschown root:postfix /etc/postfix/mariadb-email2email.cfchmod u=rw,g=r,o= /etc/postfix/mariadb-email2email.cf# Tell Postfix to use this mappingpostconf smtpd_sender_login_maps=mysql:/etc/postfix/mariadb-email2email.cf"><div></div></button></div></figure></div>
<p>What you accomplished:</p>
<ol>
<li>Create the <em>mapping</em> file</li>
<li>Fix the permissions of it so that only Postfix can read it (there is a password in it)</li>
<li>Make Postfix use that mapping for its <code dir="auto">smtpd_sender_login_maps</code> setting</li>
<li>Add <code dir="auto">smtpd_sender_restrictions=reject_sender_login_mismatch</code> to the <code dir="auto">submission</code> and <code dir="auto">submissions</code> services</li>
</ol>
<p>You can now try to send email as a different user than you are logged in. Lets us swaks again to send a spoofed email:</p>
<div class="expressive-code"><figure class="frame is-terminal has-title not-content"><figcaption class="header"><span class="title">Run this on your server</span></figcaption><pre data-language="sh"><code><div class="ec-line"><div class="code"><span style="--0:#82AAFF;--1:#3B61B0">swaks</span><span style="--0:#D6DEEB;--1:#403F53"> </span><span style="--0:#82AAFF;--1:#3B61B0">--server</span><span style="--0:#D6DEEB;--1:#403F53"> </span><span style="--0:#ECC48D;--1:#3B61B0">localhost:587</span><span style="--0:#D6DEEB;--1:#403F53"> </span><span style="--0:#F78C6C;--1:#AA0982">\</span></div></div><div class="ec-line"><div class="code"><span class="indent"> </span><span style="--0:#82AAFF;--1:#3B61B0">--from</span><span style="--0:#D6DEEB;--1:#403F53"> </span><span style="--0:#ECC48D;--1:#3B61B0">brunhilde@example.org</span><span style="--0:#D6DEEB;--1:#403F53"> </span><span style="--0:#F78C6C;--1:#AA0982">\</span></div></div><div class="ec-line"><div class="code"><span class="indent"> </span><span style="--0:#82AAFF;--1:#3B61B0">--to</span><span style="--0:#D6DEEB;--1:#403F53"> </span><span style="--0:#ECC48D;--1:#3B61B0">list@example.com</span><span style="--0:#D6DEEB;--1:#403F53"> </span><span style="--0:#F78C6C;--1:#AA0982">\</span></div></div><div class="ec-line"><div class="code"><span class="indent"> </span><span style="--0:#82AAFF;--1:#3B61B0">--tls</span><span style="--0:#D6DEEB;--1:#403F53"> </span><span style="--0:#F78C6C;--1:#AA0982">\</span></div></div><div class="ec-line"><div class="code"><span class="indent"> </span><span style="--0:#82AAFF;--1:#3B61B0">--auth-user</span><span style="--0:#D6DEEB;--1:#403F53"> </span><span style="--0:#ECC48D;--1:#3B61B0">john@example.org</span><span style="--0:#D6DEEB;--1:#403F53"> </span><span style="--0:#F78C6C;--1:#AA0982">\</span></div></div><div class="ec-line"><div class="code"><span class="indent"> </span><span style="--0:#82AAFF;--1:#3B61B0">--auth-password</span><span style="--0:#D6DEEB;--1:#403F53"> </span><span style="--0:#ECC48D;--1:#3B61B0">summersun</span></div></div></code></pre><div class="copy"><div aria-live="polite"></div><button title="Copy to clipboard" data-copied="Copied!" data-code="swaks --server localhost:587 \ --from brunhilde@example.org \ --to list@example.com \ --tls \ --auth-user john@example.org \ --auth-password summersun"><div></div></button></div></figure></div>
<p>The server should rightfully reject that wrong sender:</p>
<div class="expressive-code"><figure class="frame not-content"><figcaption class="header"></figcaption><pre data-language="plaintext"><code><div class="ec-line"><div class="code"><span style="--0:#d6deeb;--1:#403f53">~> MAIL FROM:john@example.com</span></div></div><div class="ec-line"><div class="code"><span style="--0:#d6deeb;--1:#403f53">&#x3C;~ 250 2.1.0 Ok</span></div></div><div class="ec-line"><div class="code"><span style="--0:#d6deeb;--1:#403f53">~> RCPT TO:john@example.org</span></div></div><div class="ec-line"><div class="code"><span style="--0:#d6deeb;--1:#403f53">&#x3C;~ * 553 5.7.1 john@example.com: Sender address rejected: not owned by user john@example.org</span></div></div></code></pre><div class="copy"><div aria-live="polite"></div><button title="Copy to clipboard" data-copied="Copied!" data-code="~> MAIL FROM:john@example.com<~ 250 2.1.0 Ok~> RCPT TO:john@example.org<~ * 553 5.7.1 john@example.com: Sender address rejected: not owned by user john@example.org"><div></div></button></div></figure></div>
<p>Now lets send a proper email from John:</p>
<div class="expressive-code"><figure class="frame is-terminal has-title not-content"><figcaption class="header"><span class="title">Run this on your server</span></figcaption><pre data-language="sh"><code><div class="ec-line"><div class="code"><span style="--0:#82AAFF;--1:#3B61B0">swaks</span><span style="--0:#D6DEEB;--1:#403F53"> </span><span style="--0:#82AAFF;--1:#3B61B0">--server</span><span style="--0:#D6DEEB;--1:#403F53"> </span><span style="--0:#ECC48D;--1:#3B61B0">localhost:587</span><span style="--0:#D6DEEB;--1:#403F53"> </span><span style="--0:#F78C6C;--1:#AA0982">\</span></div></div><div class="ec-line"><div class="code"><span class="indent"> </span><span style="--0:#82AAFF;--1:#3B61B0">--from</span><span style="--0:#D6DEEB;--1:#403F53"> </span><span style="--0:#ECC48D;--1:#3B61B0">john@example.org</span><span style="--0:#D6DEEB;--1:#403F53"> </span><span style="--0:#F78C6C;--1:#AA0982">\</span></div></div><div class="ec-line"><div class="code"><span class="indent"> </span><span style="--0:#82AAFF;--1:#3B61B0">--to</span><span style="--0:#D6DEEB;--1:#403F53"> </span><span style="--0:#ECC48D;--1:#3B61B0">list@example.com</span><span style="--0:#D6DEEB;--1:#403F53"> </span><span style="--0:#F78C6C;--1:#AA0982">\</span></div></div><div class="ec-line"><div class="code"><span class="indent"> </span><span style="--0:#82AAFF;--1:#3B61B0">--tls</span><span style="--0:#D6DEEB;--1:#403F53"> </span><span style="--0:#F78C6C;--1:#AA0982">\</span></div></div><div class="ec-line"><div class="code"><span class="indent"> </span><span style="--0:#82AAFF;--1:#3B61B0">--auth-user</span><span style="--0:#D6DEEB;--1:#403F53"> </span><span style="--0:#ECC48D;--1:#3B61B0">john@example.org</span><span style="--0:#D6DEEB;--1:#403F53"> </span><span style="--0:#F78C6C;--1:#AA0982">\</span></div></div><div class="ec-line"><div class="code"><span class="indent"> </span><span style="--0:#82AAFF;--1:#3B61B0">--auth-password</span><span style="--0:#D6DEEB;--1:#403F53"> </span><span style="--0:#ECC48D;--1:#3B61B0">summersun</span></div></div></code></pre><div class="copy"><div aria-live="polite"></div><button title="Copy to clipboard" data-copied="Copied!" data-code="swaks --server localhost:587 \ --from john@example.org \ --to list@example.com \ --tls \ --auth-user john@example.org \ --auth-password summersun"><div></div></button></div></figure></div>
<p>That will work:</p>
<div class="expressive-code"><figure class="frame not-content"><figcaption class="header"></figcaption><pre data-language="plaintext"><code><div class="ec-line"><div class="code"><span style="--0:#d6deeb;--1:#403f53">&#x3C;~ 250 2.0.0 Ok: queued as AFFF12292A</span></div></div></code></pre><div class="copy"><div aria-live="polite"></div><button title="Copy to clipboard" data-copied="Copied!" data-code="<~ 250 2.0.0 Ok: queued as AFFF12292A"><div></div></button></div></figure></div>
<aside aria-label="Allow aliases?" class="starlight-aside starlight-aside--tip"> <p class="starlight-aside__title" aria-hidden="true"> <svg aria-hidden="true" class="starlight-aside__icon astro-c6vsoqas" width="16" height="16" viewBox="0 0 24 24" fill="currentColor" style="--sl-icon-size: 1em;"><path fill-rule="evenodd" d="M1.44 8.855v-.001l3.527-3.516c.34-.344.802-.541 1.285-.548h6.649l.947-.947c3.07-3.07 6.207-3.072 7.62-2.868a1.821 1.821 0 0 1 1.557 1.557c.204 1.413.203 4.55-2.868 7.62l-.946.946v6.649a1.845 1.845 0 0 1-.549 1.286l-3.516 3.528a1.844 1.844 0 0 1-3.11-.944l-.858-4.275-4.52-4.52-2.31-.463-1.964-.394A1.847 1.847 0 0 1 .98 10.693a1.843 1.843 0 0 1 .46-1.838Zm5.379 2.017-3.873-.776L6.32 6.733h4.638l-4.14 4.14Zm8.403-5.655c2.459-2.46 4.856-2.463 5.89-2.33.134 1.035.13 3.432-2.329 5.891l-6.71 6.71-3.561-3.56 6.71-6.711Zm-1.318 15.837-.776-3.873 4.14-4.14v4.639l-3.364 3.374Z" clip-rule="evenodd"/><path d="M9.318 18.345a.972.972 0 0 0-1.86-.561c-.482 1.435-1.687 2.204-2.934 2.619a8.22 8.22 0 0 1-1.23.302c.062-.365.157-.79.303-1.229.415-1.247 1.184-2.452 2.62-2.935a.971.971 0 1 0-.62-1.842c-.12.04-.236.084-.35.13-2.02.828-3.012 2.588-3.493 4.033a10.383 10.383 0 0 0-.51 2.845l-.001.016v.063c0 .536.434.972.97.972H2.24a7.21 7.21 0 0 0 .878-.065c.527-.063 1.248-.19 2.02-.447 1.445-.48 3.205-1.472 4.033-3.494a5.828 5.828 0 0 0 .147-.407Z"/></svg>Allow aliases? </p> <div class="starlight-aside__content"> <p>The setup so far works for personal mails from John. But if John would be a member of a distribution list
<code dir="auto">pizza@example.org</code> then would not be able to use <code dir="auto">pizza@example.org</code> as a sender address.</p><p>If you want to allow that, create a new *.cf file with a mapping query like this:
<code dir="auto">SELECT email FROM virtual_users WHERE email='%s' UNION SELECT destination FROM virtual_aliases WHERE source='%s'</code>.</p> </div> </aside>
<aside aria-label="Check your $mydestination" class="starlight-aside starlight-aside--danger"> <p class="starlight-aside__title" aria-hidden="true"> <svg aria-hidden="true" class="starlight-aside__icon astro-c6vsoqas" width="16" height="16" viewBox="0 0 24 24" fill="currentColor" style="--sl-icon-size: 1em;"><path d="M12 7a1 1 0 0 0-1 1v4a1 1 0 0 0 2 0V8a1 1 0 0 0-1-1Zm0 8a1 1 0 1 0 0 2 1 1 0 0 0 0-2Zm9.71-7.44-5.27-5.27a1.05 1.05 0 0 0-.71-.29H8.27a1.05 1.05 0 0 0-.71.29L2.29 7.56a1.05 1.05 0 0 0-.29.71v7.46c.004.265.107.518.29.71l5.27 5.27c.192.183.445.286.71.29h7.46a1.05 1.05 0 0 0 .71-.29l5.27-5.27a1.05 1.05 0 0 0 .29-.71V8.27a1.05 1.05 0 0 0-.29-.71ZM20 15.31 15.31 20H8.69L4 15.31V8.69L8.69 4h6.62L20 8.69v6.62Z"/></svg>Check your $mydestination </p> <div class="starlight-aside__content"> <p>Please check the <code dir="auto">mydestination</code> setting in your <code dir="auto">main.cf</code> file. Make sure that it does not contain any domain name that
you also use as one of your <em>virtual domains</em>. If in doubt just set it to <code dir="auto">localhost</code>.</p> </div> </aside>
<div class="sl-heading-wrapper level-h2"><h2 id="test-it">Test it</h2><a class="sl-anchor-link" href="#test-it"><span aria-hidden="true" class="sl-anchor-icon"><svg width="16" height="16" viewBox="0 0 24 24"><path fill="currentcolor" d="m12.11 15.39-3.88 3.88a2.52 2.52 0 0 1-3.5 0 2.47 2.47 0 0 1 0-3.5l3.88-3.88a1 1 0 0 0-1.42-1.42l-3.88 3.89a4.48 4.48 0 0 0 6.33 6.33l3.89-3.88a1 1 0 1 0-1.42-1.42Zm8.58-12.08a4.49 4.49 0 0 0-6.33 0l-3.89 3.88a1 1 0 0 0 1.42 1.42l3.88-3.88a2.52 2.52 0 0 1 3.5 0 2.47 2.47 0 0 1 0 3.5l-3.88 3.88a1 1 0 1 0 1.42 1.42l3.88-3.89a4.49 4.49 0 0 0 0-6.33ZM8.83 15.17a1 1 0 0 0 1.1.22 1 1 0 0 0 .32-.22l4.92-4.92a1 1 0 0 0-1.42-1.42l-4.92 4.92a1 1 0 0 0 0 1.42Z"></path></svg></span><span class="sr-only">Section titled “Test it”</span></a></div>
<div class="sl-heading-wrapper level-h3"><h3 id="no-encryption-no-authentication">No encryption, no authentication</h3><a class="sl-anchor-link" href="#no-encryption-no-authentication"><span aria-hidden="true" class="sl-anchor-icon"><svg width="16" height="16" viewBox="0 0 24 24"><path fill="currentcolor" d="m12.11 15.39-3.88 3.88a2.52 2.52 0 0 1-3.5 0 2.47 2.47 0 0 1 0-3.5l3.88-3.88a1 1 0 0 0-1.42-1.42l-3.88 3.89a4.48 4.48 0 0 0 6.33 6.33l3.89-3.88a1 1 0 1 0-1.42-1.42Zm8.58-12.08a4.49 4.49 0 0 0-6.33 0l-3.89 3.88a1 1 0 0 0 1.42 1.42l3.88-3.88a2.52 2.52 0 0 1 3.5 0 2.47 2.47 0 0 1 0 3.5l-3.88 3.88a1 1 0 1 0 1.42 1.42l3.88-3.89a4.49 4.49 0 0 0 0-6.33ZM8.83 15.17a1 1 0 0 0 1.1.22 1 1 0 0 0 .32-.22l4.92-4.92a1 1 0 0 0-1.42-1.42l-4.92 4.92a1 1 0 0 0 0 1.42Z"></path></svg></span><span class="sr-only">Section titled “No encryption, no authentication”</span></a></div>
<p>So far you have set up the <code dir="auto">submission</code> service(s), enabled encryption and connected Postfix to Dovecot for
authentication. Lets try it. Our first test email will be sent without encryption and authentication:</p>
<div class="expressive-code"><figure class="frame is-terminal has-title not-content"><figcaption class="header"><span class="title">Run this on your server</span></figcaption><pre data-language="sh"><code><div class="ec-line"><div class="code"><span style="--0:#82AAFF;--1:#3B61B0">swaks</span><span style="--0:#D6DEEB;--1:#403F53"> </span><span style="--0:#82AAFF;--1:#3B61B0">--server</span><span style="--0:#D6DEEB;--1:#403F53"> </span><span style="--0:#ECC48D;--1:#3B61B0">localhost:587</span><span style="--0:#D6DEEB;--1:#403F53"> </span><span style="--0:#F78C6C;--1:#AA0982">\</span></div></div><div class="ec-line"><div class="code"><span class="indent"> </span><span style="--0:#82AAFF;--1:#3B61B0">--from</span><span style="--0:#D6DEEB;--1:#403F53"> </span><span style="--0:#ECC48D;--1:#3B61B0">john@example.org</span><span style="--0:#D6DEEB;--1:#403F53"> </span><span style="--0:#F78C6C;--1:#AA0982">\</span></div></div><div class="ec-line"><div class="code"><span class="indent"> </span><span style="--0:#82AAFF;--1:#3B61B0">--to</span><span style="--0:#D6DEEB;--1:#403F53"> </span><span style="--0:#ECC48D;--1:#3B61B0">lisa@example.com</span></div></div></code></pre><div class="copy"><div aria-live="polite"></div><button title="Copy to clipboard" data-copied="Copied!" data-code="swaks --server localhost:587 \ --from john@example.org \ --to lisa@example.com"><div></div></button></div></figure></div>
<p>Postfix should tell you that it will not accept an email without encryption:</p>
<div class="expressive-code"><figure class="frame not-content"><figcaption class="header"></figcaption><pre data-language="plaintext"><code><div class="ec-line"><div class="code"><span style="--0:#d6deeb;--1:#403f53">&#x3C;** 530 5.7.0 Must issue a STARTTLS command first</span></div></div></code></pre><div class="copy"><div aria-live="polite"></div><button title="Copy to clipboard" data-copied="Copied!" data-code="<** 530 5.7.0 Must issue a STARTTLS command first"><div></div></button></div></figure></div>
<div class="sl-heading-wrapper level-h3"><h3 id="with-encryption-no-authentication">With encryption, no authentication</h3><a class="sl-anchor-link" href="#with-encryption-no-authentication"><span aria-hidden="true" class="sl-anchor-icon"><svg width="16" height="16" viewBox="0 0 24 24"><path fill="currentcolor" d="m12.11 15.39-3.88 3.88a2.52 2.52 0 0 1-3.5 0 2.47 2.47 0 0 1 0-3.5l3.88-3.88a1 1 0 0 0-1.42-1.42l-3.88 3.89a4.48 4.48 0 0 0 6.33 6.33l3.89-3.88a1 1 0 1 0-1.42-1.42Zm8.58-12.08a4.49 4.49 0 0 0-6.33 0l-3.89 3.88a1 1 0 0 0 1.42 1.42l3.88-3.88a2.52 2.52 0 0 1 3.5 0 2.47 2.47 0 0 1 0 3.5l-3.88 3.88a1 1 0 1 0 1.42 1.42l3.88-3.89a4.49 4.49 0 0 0 0-6.33ZM8.83 15.17a1 1 0 0 0 1.1.22 1 1 0 0 0 .32-.22l4.92-4.92a1 1 0 0 0-1.42-1.42l-4.92 4.92a1 1 0 0 0 0 1.42Z"></path></svg></span><span class="sr-only">Section titled “With encryption, no authentication”</span></a></div>
<p>Our second attempt will <strong>enable</strong> (STARTTLS) encryption but still no authentication:</p>
<div class="expressive-code"><figure class="frame is-terminal has-title not-content"><figcaption class="header"><span class="title">Run this on your server</span></figcaption><pre data-language="sh"><code><div class="ec-line"><div class="code"><span style="--0:#82AAFF;--1:#3B61B0">swaks</span><span style="--0:#D6DEEB;--1:#403F53"> </span><span style="--0:#82AAFF;--1:#3B61B0">--server</span><span style="--0:#D6DEEB;--1:#403F53"> </span><span style="--0:#ECC48D;--1:#3B61B0">localhost:587</span><span style="--0:#D6DEEB;--1:#403F53"> </span><span style="--0:#F78C6C;--1:#AA0982">\</span></div></div><div class="ec-line"><div class="code"><span class="indent"> </span><span style="--0:#82AAFF;--1:#3B61B0">--from</span><span style="--0:#D6DEEB;--1:#403F53"> </span><span style="--0:#ECC48D;--1:#3B61B0">john@example.org</span><span style="--0:#D6DEEB;--1:#403F53"> </span><span style="--0:#F78C6C;--1:#AA0982">\</span></div></div><div class="ec-line"><div class="code"><span class="indent"> </span><span style="--0:#82AAFF;--1:#3B61B0">--to</span><span style="--0:#D6DEEB;--1:#403F53"> </span><span style="--0:#ECC48D;--1:#3B61B0">lisa@example.com</span><span style="--0:#D6DEEB;--1:#403F53"> </span><span style="--0:#F78C6C;--1:#AA0982">\</span></div></div><div class="ec-line"><div class="code"><span class="indent"> </span><span style="--0:#82AAFF;--1:#3B61B0">-tls</span></div></div></code></pre><div class="copy"><div aria-live="polite"></div><button title="Copy to clipboard" data-copied="Copied!" data-code="swaks --server localhost:587 \ --from john@example.org \ --to lisa@example.com \ -tls"><div></div></button></div></figure></div>
<p>Postfix will still reject the email because we refused to authenticate:</p>
<div class="expressive-code"><figure class="frame not-content"><figcaption class="header"></figcaption><pre data-language="plaintext"><code><div class="ec-line"><div class="code"><span class="indent"><span style="--0:#d6deeb;--1:#403f53"> </span></span><span style="--0:#d6deeb;--1:#403f53">~> MAIL FROM:&#x3C;john@example.org></span></div></div><div class="ec-line"><div class="code"><span style="--0:#d6deeb;--1:#403f53">&#x3C;~ 250 2.1.0 Ok</span></div></div><div class="ec-line"><div class="code"><span class="indent"><span style="--0:#d6deeb;--1:#403f53"> </span></span><span style="--0:#d6deeb;--1:#403f53">~> RCPT TO:&#x3C;lisa@example.com></span></div></div><div class="ec-line"><div class="code"><span style="--0:#d6deeb;--1:#403f53">&#x3C;~* 553 5.7.1 &#x3C;john@example.org>: Sender address rejected: not logged in</span></div></div></code></pre><div class="copy"><div aria-live="polite"></div><button title="Copy to clipboard" data-copied="Copied!" data-code=" ~> MAIL FROM:<john@example.org><~ 250 2.1.0 Ok ~> RCPT TO:<lisa@example.com><~* 553 5.7.1 <john@example.org>: Sender address rejected: not logged in"><div></div></button></div></figure></div>
<div class="sl-heading-wrapper level-h3"><h3 id="with-encryption-and-authentication">With encryption and authentication</h3><a class="sl-anchor-link" href="#with-encryption-and-authentication"><span aria-hidden="true" class="sl-anchor-icon"><svg width="16" height="16" viewBox="0 0 24 24"><path fill="currentcolor" d="m12.11 15.39-3.88 3.88a2.52 2.52 0 0 1-3.5 0 2.47 2.47 0 0 1 0-3.5l3.88-3.88a1 1 0 0 0-1.42-1.42l-3.88 3.89a4.48 4.48 0 0 0 6.33 6.33l3.89-3.88a1 1 0 1 0-1.42-1.42Zm8.58-12.08a4.49 4.49 0 0 0-6.33 0l-3.89 3.88a1 1 0 0 0 1.42 1.42l3.88-3.88a2.52 2.52 0 0 1 3.5 0 2.47 2.47 0 0 1 0 3.5l-3.88 3.88a1 1 0 1 0 1.42 1.42l3.88-3.89a4.49 4.49 0 0 0 0-6.33ZM8.83 15.17a1 1 0 0 0 1.1.22 1 1 0 0 0 .32-.22l4.92-4.92a1 1 0 0 0-1.42-1.42l-4.92 4.92a1 1 0 0 0 0 1.42Z"></path></svg></span><span class="sr-only">Section titled “With encryption and authentication”</span></a></div>
<div class="expressive-code"><figure class="frame is-terminal has-title not-content"><figcaption class="header"><span class="title">Run this on your server</span></figcaption><pre data-language="sh"><code><div class="ec-line"><div class="code"><span style="--0:#82AAFF;--1:#3B61B0">swaks</span><span style="--0:#D6DEEB;--1:#403F53"> </span><span style="--0:#82AAFF;--1:#3B61B0">--server</span><span style="--0:#D6DEEB;--1:#403F53"> </span><span style="--0:#ECC48D;--1:#3B61B0">localhost:587</span><span style="--0:#D6DEEB;--1:#403F53"> </span><span style="--0:#F78C6C;--1:#AA0982">\</span></div></div><div class="ec-line"><div class="code"><span class="indent"> </span><span style="--0:#82AAFF;--1:#3B61B0">--from</span><span style="--0:#D6DEEB;--1:#403F53"> </span><span style="--0:#ECC48D;--1:#3B61B0">john@example.org</span><span style="--0:#D6DEEB;--1:#403F53"> </span><span style="--0:#F78C6C;--1:#AA0982">\</span></div></div><div class="ec-line"><div class="code"><span class="indent"> </span><span style="--0:#82AAFF;--1:#3B61B0">--to</span><span style="--0:#D6DEEB;--1:#403F53"> </span><span style="--0:#ECC48D;--1:#3B61B0">lisa@example.com</span><span style="--0:#D6DEEB;--1:#403F53"> </span><span style="--0:#F78C6C;--1:#AA0982">\</span></div></div><div class="ec-line"><div class="code"><span class="indent"> </span><span style="--0:#82AAFF;--1:#3B61B0">-tls</span><span style="--0:#D6DEEB;--1:#403F53"> </span><span style="--0:#F78C6C;--1:#AA0982">\</span></div></div><div class="ec-line"><div class="code"><span class="indent"> </span><span style="--0:#82AAFF;--1:#3B61B0">--auth-user</span><span style="--0:#D6DEEB;--1:#403F53"> </span><span style="--0:#ECC48D;--1:#3B61B0">john@example.org</span><span style="--0:#D6DEEB;--1:#403F53"> </span><span style="--0:#F78C6C;--1:#AA0982">\</span></div></div><div class="ec-line"><div class="code"><span class="indent"> </span><span style="--0:#82AAFF;--1:#3B61B0">--auth-password</span><span style="--0:#D6DEEB;--1:#403F53"> </span><span style="--0:#ECC48D;--1:#3B61B0">summersun</span></div></div></code></pre><div class="copy"><div aria-live="polite"></div><button title="Copy to clipboard" data-copied="Copied!" data-code="swaks --server localhost:587 \ --from john@example.org \ --to lisa@example.com \ -tls \ --auth-user john@example.org \ --auth-password summersun"><div></div></button></div></figure></div>
<p>Also give the <code dir="auto">submissions</code> service a try:</p>
<div class="expressive-code"><figure class="frame is-terminal has-title not-content"><figcaption class="header"><span class="title">Run this on your server</span></figcaption><pre data-language="sh"><code><div class="ec-line"><div class="code"><span style="--0:#82AAFF;--1:#3B61B0">swaks</span><span style="--0:#D6DEEB;--1:#403F53"> </span><span style="--0:#82AAFF;--1:#3B61B0">--server</span><span style="--0:#D6DEEB;--1:#403F53"> </span><span style="--0:#ECC48D;--1:#3B61B0">localhost:465</span><span style="--0:#D6DEEB;--1:#403F53"> </span><span style="--0:#F78C6C;--1:#AA0982">\</span></div></div><div class="ec-line"><div class="code"><span class="indent"> </span><span style="--0:#82AAFF;--1:#3B61B0">--from</span><span style="--0:#D6DEEB;--1:#403F53"> </span><span style="--0:#ECC48D;--1:#3B61B0">john@example.org</span><span style="--0:#D6DEEB;--1:#403F53"> </span><span style="--0:#F78C6C;--1:#AA0982">\</span></div></div><div class="ec-line"><div class="code"><span class="indent"> </span><span style="--0:#82AAFF;--1:#3B61B0">--to</span><span style="--0:#D6DEEB;--1:#403F53"> </span><span style="--0:#ECC48D;--1:#3B61B0">lisa@example.com</span><span style="--0:#D6DEEB;--1:#403F53"> </span><span style="--0:#F78C6C;--1:#AA0982">\</span></div></div><div class="ec-line"><div class="code"><span class="indent"> </span><span style="--0:#82AAFF;--1:#3B61B0">--tls-on-connect</span><span style="--0:#D6DEEB;--1:#403F53"> </span><span style="--0:#F78C6C;--1:#AA0982">\</span></div></div><div class="ec-line"><div class="code"><span class="indent"> </span><span style="--0:#82AAFF;--1:#3B61B0">--auth-user</span><span style="--0:#D6DEEB;--1:#403F53"> </span><span style="--0:#ECC48D;--1:#3B61B0">john@example.org</span><span style="--0:#D6DEEB;--1:#403F53"> </span><span style="--0:#F78C6C;--1:#AA0982">\</span></div></div><div class="ec-line"><div class="code"><span class="indent"> </span><span style="--0:#82AAFF;--1:#3B61B0">--auth-password</span><span style="--0:#D6DEEB;--1:#403F53"> </span><span style="--0:#ECC48D;--1:#3B61B0">summersun</span></div></div></code></pre><div class="copy"><div aria-live="polite"></div><button title="Copy to clipboard" data-copied="Copied!" data-code="swaks --server localhost:465 \ --from john@example.org \ --to lisa@example.com \ --tls-on-connect \ --auth-user john@example.org \ --auth-password summersun"><div></div></button></div></figure></div>
<p>Did you notice the difference? The second time we used port <strong>465</strong> and instead of the <code dir="auto">-tls</code> option (which uses
STARTTLS to switch to encryption) we use <code dir="auto">--tls-on-connect</code> that uses TLS right from from the start.</p>
<p>Put simply, a mail server will accept an email if…</p>
<ul>
<li>it is responsible for handling mail for the recipients domain (the domain is listed as a virtual domain), or</li>
<li>the sender has authenticated successfully with a valid username and password.</li>
</ul>
<p>I have created two illustrations to explain it.</p>
<div class="sl-heading-wrapper level-h2"><h2 id="gory-details-of-authentication">Gory details of authentication</h2><a class="sl-anchor-link" href="#gory-details-of-authentication"><span aria-hidden="true" class="sl-anchor-icon"><svg width="16" height="16" viewBox="0 0 24 24"><path fill="currentcolor" d="m12.11 15.39-3.88 3.88a2.52 2.52 0 0 1-3.5 0 2.47 2.47 0 0 1 0-3.5l3.88-3.88a1 1 0 0 0-1.42-1.42l-3.88 3.89a4.48 4.48 0 0 0 6.33 6.33l3.89-3.88a1 1 0 1 0-1.42-1.42Zm8.58-12.08a4.49 4.49 0 0 0-6.33 0l-3.89 3.88a1 1 0 0 0 1.42 1.42l3.88-3.88a2.52 2.52 0 0 1 3.5 0 2.47 2.47 0 0 1 0 3.5l-3.88 3.88a1 1 0 1 0 1.42 1.42l3.88-3.89a4.49 4.49 0 0 0 0-6.33ZM8.83 15.17a1 1 0 0 0 1.1.22 1 1 0 0 0 .32-.22l4.92-4.92a1 1 0 0 0-1.42-1.42l-4.92 4.92a1 1 0 0 0 0 1.42Z"></path></svg></span><span class="sr-only">Section titled “Gory details of authentication”</span></a></div>
<aside aria-label="For science" class="starlight-aside starlight-aside--tip"> <p class="starlight-aside__title" aria-hidden="true"> <svg aria-hidden="true" class="starlight-aside__icon astro-c6vsoqas" width="16" height="16" viewBox="0 0 24 24" fill="currentColor" style="--sl-icon-size: 1em;"><path fill-rule="evenodd" d="M1.44 8.855v-.001l3.527-3.516c.34-.344.802-.541 1.285-.548h6.649l.947-.947c3.07-3.07 6.207-3.072 7.62-2.868a1.821 1.821 0 0 1 1.557 1.557c.204 1.413.203 4.55-2.868 7.62l-.946.946v6.649a1.845 1.845 0 0 1-.549 1.286l-3.516 3.528a1.844 1.844 0 0 1-3.11-.944l-.858-4.275-4.52-4.52-2.31-.463-1.964-.394A1.847 1.847 0 0 1 .98 10.693a1.843 1.843 0 0 1 .46-1.838Zm5.379 2.017-3.873-.776L6.32 6.733h4.638l-4.14 4.14Zm8.403-5.655c2.459-2.46 4.856-2.463 5.89-2.33.134 1.035.13 3.432-2.329 5.891l-6.71 6.71-3.561-3.56 6.71-6.711Zm-1.318 15.837-.776-3.873 4.14-4.14v4.639l-3.364 3.374Z" clip-rule="evenodd"/><path d="M9.318 18.345a.972.972 0 0 0-1.86-.561c-.482 1.435-1.687 2.204-2.934 2.619a8.22 8.22 0 0 1-1.23.302c.062-.365.157-.79.303-1.229.415-1.247 1.184-2.452 2.62-2.935a.971.971 0 1 0-.62-1.842c-.12.04-.236.084-.35.13-2.02.828-3.012 2.588-3.493 4.033a10.383 10.383 0 0 0-.51 2.845l-.001.016v.063c0 .536.434.972.97.972H2.24a7.21 7.21 0 0 0 .878-.065c.527-.063 1.248-.19 2.02-.447 1.445-.48 3.205-1.472 4.033-3.494a5.828 5.828 0 0 0 .147-.407Z"/></svg>For science </p> <div class="starlight-aside__content"> <p>This section is just here to show you the communication that happens on the network when relaying an email. It is no
mandatory reading matter. But relaying is a complex matter where things can go wrong. So taking a peek under hood may
improve your understanding.</p> </div> </aside>
<div class="expressive-code"><figure class="frame not-content"><figcaption class="header"></figcaption><pre data-language="plaintext"><code><div class="ec-line"><div class="code"><span style="--0:#d6deeb;--1:#403f53">telnet localhost submission</span></div></div></code></pre><div class="copy"><div aria-live="polite"></div><button title="Copy to clipboard" data-copied="Copied!" data-code="telnet localhost submission"><div></div></button></div></figure></div>
<p>The server will let you in:</p>
<div class="expressive-code"><figure class="frame not-content"><figcaption class="header"></figcaption><pre data-language="plaintext"><code><div class="ec-line"><div class="code"><span style="--0:#d6deeb;--1:#403f53">Trying 127.0.0.1…</span></div></div><div class="ec-line"><div class="code"><span style="--0:#d6deeb;--1:#403f53">Connected to localhost.</span></div></div><div class="ec-line"><div class="code"><span style="--0:#d6deeb;--1:#403f53">Escape character is '^]'.</span></div></div><div class="ec-line"><div class="code"><span style="--0:#d6deeb;--1:#403f53">220 webmail ESMTP Postfix (Debian)</span></div></div></code></pre><div class="copy"><div aria-live="polite"></div><button title="Copy to clipboard" data-copied="Copied!" data-code="Trying 127.0.0.1…Connected to localhost.Escape character is &#x27;^]&#x27;.220 webmail ESMTP Postfix (Debian)"><div></div></button></div></figure></div>
<p>Say hello:</p>
<div class="expressive-code"><figure class="frame not-content"><figcaption class="header"></figcaption><pre data-language="plaintext"><code><div class="ec-line"><div class="code"><span style="--0:#d6deeb;--1:#403f53">ehlo example.com</span></div></div></code></pre><div class="copy"><div aria-live="polite"></div><button title="Copy to clipboard" data-copied="Copied!" data-code="ehlo example.com"><div></div></button></div></figure></div>
<p>Postfix will present a list of features that are available for you:</p>
<div class="expressive-code"><figure class="frame not-content"><figcaption class="header"></figcaption><pre data-language="plaintext"><code><div class="ec-line"><div class="code"><span style="--0:#d6deeb;--1:#403f53">250-mailtest</span></div></div><div class="ec-line"><div class="code"><span style="--0:#d6deeb;--1:#403f53">250-PIPELINING</span></div></div><div class="ec-line"><div class="code"><span style="--0:#d6deeb;--1:#403f53">250-SIZE 10240000</span></div></div><div class="ec-line"><div class="code"><span style="--0:#d6deeb;--1:#403f53">250-VRFY</span></div></div><div class="ec-line"><div class="code"><span style="--0:#d6deeb;--1:#403f53">250-ETRN</span></div></div><div class="ec-line"><div class="code"><span style="--0:#d6deeb;--1:#403f53">250-STARTTLS</span></div></div><div class="ec-line"><div class="code"><span style="--0:#d6deeb;--1:#403f53">250-ENHANCEDSTATUSCODES</span></div></div><div class="ec-line"><div class="code"><span style="--0:#d6deeb;--1:#403f53">250-8BITMIME</span></div></div><div class="ec-line"><div class="code"><span style="--0:#d6deeb;--1:#403f53">250-DSN</span></div></div><div class="ec-line"><div class="code"><span style="--0:#d6deeb;--1:#403f53">250-SMTPUTF8</span></div></div><div class="ec-line"><div class="code"><span style="--0:#d6deeb;--1:#403f53">250 CHUNKING</span></div></div></code></pre><div class="copy"><div aria-live="polite"></div><button title="Copy to clipboard" data-copied="Copied!" data-code="250-mailtest250-PIPELINING250-SIZE 10240000250-VRFY250-ETRN250-STARTTLS250-ENHANCEDSTATUSCODES250-8BITMIME250-DSN250-SMTPUTF8250 CHUNKING"><div></div></button></div></figure></div>
<p>Let me briefly explain what these lines mean:</p>
<ul>
<li>PIPELINING<br>
This is a feature to speed up SMTP communication. Usually the remote system has to wait for a response to every
command it sends. Pipelining allows the remote server to send several commands in a batch without waiting for a
response. Postfix will just store these commands and execute them one by one. If you told Postfix to forbid pipelining
it would disconnect the remote server when it tries to send bulks of commands without waiting for the proper reply. It
is mainly a feature against rogue senders.</li>
<li>SIZE 10240000<br>
The remote server is allowed to send emails up to 10 MB large. This has long been a common maximum size for emails.
However nowadays 40 MB or even more are more common sizes because emails have grown larger. Some mail servers even
allow 100 MB.</li>
<li>VRFY<br>
Allows remote servers to verify a given name or email address. For example the remote server could send “VRFY john”
and your server might respond <code dir="auto">250 Jo You can skip it for now.hn Doe &#x3C;john@example.org></code>. It can be used to verify
that a certain recipient email address is deliverable</li>
<li>ETRN<br>
A command that a remote system can send to flush the Postfix queue of mails for a certain domain. It can be used if
the remote system had technical problems and failed to receive email for a while. Then it could send an ETRN command
to make your server start sending outstanding emails for that domain. It is rarely used.</li>
<li>STARTTLS<br>
This tells the remote system that it might start switching from this unencrypted to an encrypted connection by
sending the “STARTTLS” command. It will then start negotiating a TLS-encrypted connection. You could compare it to an
HTTP connection that suddenly switches over to an encrypted HTTPS connection. The advantage is that you can start
talking SMTP on TCP port 25 and dont have to open up a second TCP port like 465 which is the “SSMTP” (secure SMTP)
port and only accepts encrypted connections.</li>
<li>ENHANCEDSTATUSCODES<br>
This enables more three-digit return codes for various conditions. See the
<a href="http://tools.ietf.org/html/rfc2034">RFC2034</a> if you are curious.</li>
<li>8BITMIME<br>
In ancient times SMTP only processed 7-bit characters. You couldnt transfer special characters like “Ä” or “ß”
without special encoding. 8BITMIME allows a transmission of emails using 8-bit characters. Still many emails are
specially encoded using ISO8859-1 or UTF-8.</li>
<li>DSN<br>
It enables DSNs (delivery status notofications) that allows the sender to control the messages that Postfix creates
when an email could not be delivered as intended.</li>
<li>SMTPUTF8<br>
In addition to 8BITMIME you can use UTF8 encoded characters in header fields.</li>
<li>CHUNKING<br>
This feature (described in <a href="https://tools.ietf.org/html/rfc3030">RFC 3030</a>) makes sending of large emails more
efficient.</li>
</ul>
<p>However one important line is missing here that would allow us to send our username and password:</p>
<div class="expressive-code"><figure class="frame not-content"><figcaption class="header"></figcaption><pre data-language="plaintext"><code><div class="ec-line"><div class="code"><span style="--0:#d6deeb;--1:#403f53">250-AUTH PLAIN LOGIN</span></div></div></code></pre><div class="copy"><div aria-live="polite"></div><button title="Copy to clipboard" data-copied="Copied!" data-code="250-AUTH PLAIN LOGIN"><div></div></button></div></figure></div>
<p>We told Postfix to only allow authentication when the connection is encrypted. So we are not offered authentication over
this plain text connection. Thats what we want.</p>
<p>Are you still connected? Okay, good. So we need an encrypted connection using TLS. Using the STARTTLS feature we can
switch over from unencrypted to encrypted without having to reconnect. Enter…</p>
<div class="expressive-code"><figure class="frame not-content"><figcaption class="header"></figcaption><pre data-language="plaintext"><code><div class="ec-line"><div class="code"><span style="--0:#d6deeb;--1:#403f53">STARTTLS</span></div></div></code></pre><div class="copy"><div aria-live="polite"></div><button title="Copy to clipboard" data-copied="Copied!" data-code="STARTTLS"><div></div></button></div></figure></div>
<p>And the server replies:</p>
<div class="expressive-code"><figure class="frame not-content"><figcaption class="header"></figcaption><pre data-language="plaintext"><code><div class="ec-line"><div class="code"><span style="--0:#d6deeb;--1:#403f53">220 2.0.0 Ready to start TLS</span></div></div></code></pre><div class="copy"><div aria-live="polite"></div><button title="Copy to clipboard" data-copied="Copied!" data-code="220 2.0.0 Ready to start TLS"><div></div></button></div></figure></div>
<p>However now its getting weird because you would have to speak TLS encryption which is not a language that humans speak.
So lets quit this using the “QUIT” command and do that differently:</p>
<div class="expressive-code"><figure class="frame not-content"><figcaption class="header"></figcaption><pre data-language="plaintext"><code><div class="ec-line"><div class="code"><span style="--0:#d6deeb;--1:#403f53">QUIT</span></div></div></code></pre><div class="copy"><div aria-live="polite"></div><button title="Copy to clipboard" data-copied="Copied!" data-code="QUIT"><div></div></button></div></figure></div>
<p>We can use OpenSSL to help us with the decryption. Run:</p>
<div class="expressive-code"><figure class="frame not-content"><figcaption class="header"></figcaption><pre data-language="plaintext"><code><div class="ec-line"><div class="code"><span style="--0:#d6deeb;--1:#403f53">openssl s_client -connect localhost:submission -starttls smtp</span></div></div></code></pre><div class="copy"><div aria-live="polite"></div><button title="Copy to clipboard" data-copied="Copied!" data-code="openssl s_client -connect localhost:submission -starttls smtp"><div></div></button></div></figure></div>
<p>You will see a lot of output. OpenSSL has connected to TCP port 25 and issued a STARTTLS command to switch to an
encrypted connection. So whatever you type now will get encrypted. Enter:</p>
<div class="expressive-code"><figure class="frame not-content"><figcaption class="header"></figcaption><pre data-language="plaintext"><code><div class="ec-line"><div class="code"><span style="--0:#d6deeb;--1:#403f53">EHLO example.com</span></div></div></code></pre><div class="copy"><div aria-live="polite"></div><button title="Copy to clipboard" data-copied="Copied!" data-code="EHLO example.com"><div></div></button></div></figure></div>
<p>And Postfix will send a list of capabilities that will look like this:</p>
<div class="expressive-code"><figure class="frame not-content"><figcaption class="header"></figcaption><pre data-language="plaintext"><code><div class="ec-line"><div class="code"><span style="--0:#d6deeb;--1:#403f53">250-PIPELINING</span></div></div><div class="ec-line"><div class="code"><span style="--0:#d6deeb;--1:#403f53">250-SIZE 10240000</span></div></div><div class="ec-line"><div class="code"><span style="--0:#d6deeb;--1:#403f53">250-VRFY</span></div></div><div class="ec-line"><div class="code"><span style="--0:#d6deeb;--1:#403f53">250-ETRN</span></div></div><div class="ec-line"><div class="code"><span style="--0:#d6deeb;--1:#403f53">250-AUTH PLAIN LOGIN</span></div></div><div class="ec-line"><div class="code"><span style="--0:#d6deeb;--1:#403f53">250-ENHANCEDSTATUSCODES</span></div></div><div class="ec-line"><div class="code"><span style="--0:#d6deeb;--1:#403f53">250-8BITMIME</span></div></div><div class="ec-line"><div class="code"><span style="--0:#d6deeb;--1:#403f53">250 DSN</span></div></div></code></pre><div class="copy"><div aria-live="polite"></div><button title="Copy to clipboard" data-copied="Copied!" data-code="250-PIPELINING250-SIZE 10240000250-VRFY250-ETRN250-AUTH PLAIN LOGIN250-ENHANCEDSTATUSCODES250-8BITMIME250 DSN"><div></div></button></div></figure></div>
<p>And now that we are using an encrypted connection Postfix offers us to authenticate. So let us send the authentication
string with a Base64-encoded password:</p>
<div class="expressive-code"><figure class="frame not-content"><figcaption class="header"></figcaption><pre data-language="plaintext"><code><div class="ec-line"><div class="code"><span style="--0:#d6deeb;--1:#403f53">AUTH PLAIN AGpvaG5AZXhhbXBsZS5vcmcAc3VtbWVyc3Vu</span></div></div></code></pre><div class="copy"><div aria-live="polite"></div><button title="Copy to clipboard" data-copied="Copied!" data-code="AUTH PLAIN AGpvaG5AZXhhbXBsZS5vcmcAc3VtbWVyc3Vu"><div></div></button></div></figure></div>
<p>Unless you have changed Johns password to something else than “summersun” the server should accept that authentication:</p>
<div class="expressive-code"><figure class="frame not-content"><figcaption class="header"></figcaption><pre data-language="plaintext"><code><div class="ec-line"><div class="code"><span style="--0:#d6deeb;--1:#403f53">235 2.7.0 Authentication successful</span></div></div></code></pre><div class="copy"><div aria-live="polite"></div><button title="Copy to clipboard" data-copied="Copied!" data-code="235 2.7.0 Authentication successful"><div></div></button></div></figure></div>
<p>Excellent. You are logged in through SMTP. You could now send an email to be forwarded to another mail server. I just
wanted to show that authentication works if you use an encrypted connection.</p>
<p>Disconnect from Postfix:</p>
<div class="expressive-code"><figure class="frame not-content"><figcaption class="header"></figcaption><pre data-language="plaintext"><code><div class="ec-line"><div class="code"><span style="--0:#d6deeb;--1:#403f53">QUIT</span></div></div></code></pre><div class="copy"><div aria-live="polite"></div><button title="Copy to clipboard" data-copied="Copied!" data-code="QUIT"><div></div></button></div></figure></div>
<p>Authentication works. Well done.</p>
<p>Wait a minute. What was that crazy cryptic string? There was no username and password in it. Was it encrypted?</p>
<p>No, that was no encryption. It was merely an <em>encoded</em> version of the username and password. Why that? Well usually in
SMTP you can only transfer ASCII characters. But the password may contain special characters which are not covered by
ASCII. So in the <em>PLAIN</em> method that information is <a href="https://en.wikipedia.org/wiki/Base64">Base64</a> encoded.</p>
<p>What is actually converted to Base64…</p>
<div class="expressive-code"><figure class="frame not-content"><figcaption class="header"></figcaption><pre data-language="plaintext"><code><div class="ec-line"><div class="code"><span style="--0:#d6deeb;--1:#403f53">NULL-BYTE + USERNAME + NULL-BYTE + PASSWORD</span></div></div></code></pre><div class="copy"><div aria-live="polite"></div><button title="Copy to clipboard" data-copied="Copied!" data-code="NULL-BYTE + USERNAME + NULL-BYTE + PASSWORD"><div></div></button></div></figure></div>
<p>So for Johns case you can easily create the Base64 string using:</p>
<div class="expressive-code"><figure class="frame not-content"><figcaption class="header"></figcaption><pre data-language="plaintext"><code><div class="ec-line"><div class="code"><span style="--0:#d6deeb;--1:#403f53">printf '\0john@example.org\0summersun' | base64</span></div></div></code></pre><div class="copy"><div aria-live="polite"></div><button title="Copy to clipboard" data-copied="Copied!" data-code="printf &#x27;\0john@example.org\0summersun&#x27; | base64"><div></div></button></div></figure></div>
<p>As a result you will get the exact same string you used above with “AUTH PLAIN”.</p>
<section data-footnotes="" class="footnotes"><div class="sl-heading-wrapper level-h2"><h2 class="sr-only" id="footnote-label">Footnotes</h2><a class="sl-anchor-link" href="#footnote-label"><span aria-hidden="true" class="sl-anchor-icon"><svg width="16" height="16" viewBox="0 0 24 24"><path fill="currentcolor" d="m12.11 15.39-3.88 3.88a2.52 2.52 0 0 1-3.5 0 2.47 2.47 0 0 1 0-3.5l3.88-3.88a1 1 0 0 0-1.42-1.42l-3.88 3.89a4.48 4.48 0 0 0 6.33 6.33l3.89-3.88a1 1 0 1 0-1.42-1.42Zm8.58-12.08a4.49 4.49 0 0 0-6.33 0l-3.89 3.88a1 1 0 0 0 1.42 1.42l3.88-3.88a2.52 2.52 0 0 1 3.5 0 2.47 2.47 0 0 1 0 3.5l-3.88 3.88a1 1 0 1 0 1.42 1.42l3.88-3.89a4.49 4.49 0 0 0 0-6.33ZM8.83 15.17a1 1 0 0 0 1.1.22 1 1 0 0 0 .32-.22l4.92-4.92a1 1 0 0 0-1.42-1.42l-4.92 4.92a1 1 0 0 0 0 1.42Z"></path></svg></span><span class="sr-only">Section titled “Footnotes”</span></a></div>
<ol>
<li id="user-content-fn-1">
<p>In previous guides I recommended to set <code dir="auto">smtpd_sasl_auth_enable=yes</code> globally in the <code dir="auto">main.cf</code>. That would indeed
enable authentication on port 25. For a clearer separation I now rather suggest to leave it out (default is <code dir="auto">no</code>)
and just enable authentication on the <code dir="auto">submission</code> service in the <code dir="auto">master.cf</code> file. A possible scenario that
required this setting to be <code dir="auto">on</code> would be if your server were a relay (<em>smarthost</em>) for other servers using
authentication.. <a href="#user-content-fnref-1" data-footnote-backref="" aria-label="Back to reference 1" class="data-footnote-backref"></a> <a href="#user-content-fnref-1-2" data-footnote-backref="" aria-label="Back to reference 1-2" class="data-footnote-backref"><sup>2</sup></a></p>
</li>
</ol>
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