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# TLS certificate

Todo list for receiving emails

1. 1

   DNS records point to your server

2. 2

   Get a certificate from Let's Encrypt

3. 3

   Postfix fetches information from MariaDB

4. 4

   Dovecot fetches information from MariaDB

5. 5

   Postfix hands over emails to Dovecot

6. 6

   Dovecot saves the email to disk

Your mail server can’t work properly without a valid TLS certificate. It will be used in three places:

1. the webmail interface (driven by the Apache web server)
2. Postfix (to encrypt SMTP communication with other mail servers on the internet where possible)
3. Dovecot (to encrypt IMAP/POP3 communication)

Your mail server will be able to host email addresses for many domains. But a TLS certificate is usually issued for a single FQDN like `mail.example.org`. That common name is an important part of the certificate. It must match the name your users use to talk to the server.

Correct: Your mail server is addressed as `mail.example.org`. The TLS certificate was also issued to `mail.example.org`.

Wrong: Your mail server is addressed as `smtp01.example.com`. But the TLS certificate was issued to `mail.example.org`.

So choose an FQDN for your mail server. For example I use `webmail.workaround.org` for my personal server.

## Certbot

Section titled “Certbot”

You already have the `certbot` software and the `apache2` web server installed. Requesting a certificate from Let’s Encrypt is as simple as running the next command.

Important

Please replace `mail.example.org` by the FQDN you chose.

Run this on your server

    certbot --apache --register-unsafely-without-email --agree-tos -d mail.example.org

Click here to learn what the options mean…

- --apache: Use the HTTP challenge to verify that you are actually in control of the FQDN. The certbot plugin will add some configuration to intercept HTTP requests to `/.well-known/acme-challenge/…` on your web server. This is a fully automatic process that does not even have to stop your web server.
- --register-unsafely-without-email: Do not ask for an email address to send to Let’s Encrypt. Before June 2025 Let’s Encrypt used that address to inform you if your certificate is about to expire and the automatic renewal failed. Nowadays they do not need an email any more. But for historical reasons they still ask for it by default.
- --agree-tos: Confirm that you have read Let’s Encrypt’s terms and conditions. Let’s be honest – nobody reads that.
- -d: The domain that the certificate will be issued to. This has to be the exact FQDN of your server.

The output should look like:

    Saving debug log to /var/log/letsencrypt/letsencrypt.logRequesting a certificate for mail.example.org
    Successfully received certificate.Certificate is saved at: /etc/letsencrypt/live/mail.example.org/fullchain.pemKey is saved at:         /etc/letsencrypt/live/mail.example.org/privkey.pemThis certificate expires on 2025-11-22.These files will be updated when the certificate renews.Certbot has set up a scheduled task to automatically renew this certificate in the background.
    Deploying certificateSuccessfully deployed certificate for mail.example.org to /etc/apache2/sites-available/000-default-le-ssl.confCongratulations! You have successfully enabled HTTPS on https://mail.example.org
    - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -If you like Certbot, please consider supporting our work by: * Donating to ISRG / Let's Encrypt:   https://letsencrypt.org/donate * Donating to EFF:                    https://eff.org/donate-le- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -

Click here to learn which files have been created…

In /etc/letsencrypt/live/mail.example.org you will find a couple of files now:

- cert.pem: the certificate file
- chain.pem: the chaining or intermediate certificate. This certificate provides information how the LetsEncrypt certificates are linked to other known certificate authorities. It is generally a good idea to always send this certificate along with your own for clients who may not know LetsEncrypt properly yet.
- fullchain.pem: this file contains a concatenation of the cert.pem and the chain.pem. This is the preferred file to use when a piece of software asks where to find the certificate.
- privkey.pem: the private key file. Keep it secret.

Well, actually those are not files but symbolic links pointing to the “archive” directory. But don’t worry about that.

## Renewal

Section titled “Renewal”

Your new certificate is only valid for 3 months. Fortunately the renewal is happening automatically. A systemd timer checks your certificate frequently and triggers a renewal one month before it expires.

It is important that services that use the certificate load the new certificate after an automatic renewal. This shell command will create a file `/etc/letsencrypt/cli.ini` that handles it:

Run this on your server

    cat > /etc/letsencrypt/cli.ini << 'EOF'# Restart services after renewing a certificatepost-hook = systemctl reload postfix dovecot apache2
    # Because we are using logrotate for greater flexibility, disable the# internal certbot logrotation.max-log-backups = 0
    # Adjust interactive output regarding automated renewalpreconfigured-renewal = TrueEOF

Perfect. You won’t have to worry about the certificate again.

Click here to learn how the renewal works…

These commands will show you the definition and status of the timer and the renewal service:

Run this on your server

    systemctl status certbot.timersystemctl cat certbot.timersystemctl status certbot.servicesystemctl cat certbot.service

Systemd timers are similar to cron jobs. They require more work to set up but have some nice features and make you look cooler.