continued writing the section on receiving mails
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@ -76,4 +76,139 @@ servers may be more likely so flag your sent emails as spam if you do not have a
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## Postfix
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…
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Now that other mail servers can locate your mail server, the next step is to make sure it can receive and process
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incoming emails. This task is handled by Postfix, which communicates using the SMTP protocol.
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Postfix knows if an email address is valid by checking for…
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- Virtual Domains (am I responsible for this domain?)
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- Virtual Aliases (do I have to redirect this address to another address?)
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- Virtual Mailboxes (do I have a valid mailbox to store the email to?)
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### Virtual Domains
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There are different ways for Postfix to get that information. From text files, LDAP, MongoDB or PostgreSQL. Or via
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MariaDB – which is what we will use. But whatever kind of data source you configure, Postfix needs the information as a
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_mapping_. Consider it as a _question_ and an _answer_.
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- Question: is `example.org` one of our virtual domains?
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- Answer: yes
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If the information is **not** available you just get no answer:
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- Question: is `example.net` one of our virtual domains?
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- Answer: (no answer)
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So Postfix hast to ask MariaDB that question:
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```sql
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SELECT "yes" FROM virtual_domains WHERE name='example.net'
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```
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Run the following code in your shell to create a configuration file creating that mapping:
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```sh
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cat > /etc/postfix/virtual-mailbox-domains.cf << EOF
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user = mailserver
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password = SECOND-PASSWORD-HERE
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hosts = 127.0.0.1
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dbname = mailserver
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query = SELECT 1 FROM virtual_domains WHERE name='%s'
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EOF
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```
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This has created a configuration file called `/etc/postfix/virtual-mailbox-domains.cf`. The `user`, `password`, `hosts`
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and `dbname` definitions tell Postfix how to connect to the database. And the `query` asks the _question_ if a certain
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domain is present in the database table `virtual_domains`. Using `SELECT 1` we return just the number 1 if such an entry
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was found. In fact it does not matter what we return. Any _answer_ is fine.
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Don't forget to replace the `SECOND-PASSWORD-HERE` by your own password for the `mailserver` user.
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### Virtual Aliases
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Now let's deal with aliases. As shown earlier an alias redirects an email to another email address. So the _question_
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and _answer_ game goes like this:
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- Question: is there an alias for `jack@example.org`?
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- Answer: `john@example.org`
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Postfix would then check if there are further aliases:
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- Question: is there an alias for `john@example.org`?
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- Answer: (no answer)
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Apparently, there aren't any. So the email will go to `john@example.org`.
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Run this code to create the appropriate mapping file:
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```sh
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cat > /etc/postfix/virtual-alias-maps.cf << EOF
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user = mailserver
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password = SECOND-PASSWORD-HERE
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hosts = 127.0.0.1
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dbname = mailserver
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query = SELECT destination FROM virtual_aliases WHERE source='%s'
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EOF
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```
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The query now gets all the `destination` email addresses from the database for a certain `source` email address.
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### Virtual Mailboxes
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The last mapping we need is a query to find valid mailboxes. Without further ado:
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```sh
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cat > /etc/postfix/virtual-mailbox-maps.cf << EOF
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user = mailserver
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password = SECOND-PASSWORD-HERE
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hosts = 127.0.0.1
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dbname = mailserver
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query = SELECT 1 FROM virtual_mailboxes WHERE email='%s'
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EOF
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```
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Just the same story as with virtual domains. The database returns `1` if there is a `virtual_mailboxes` record matching
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that specific email address.
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Actually the _answer_ is usually not `1` but the path to the mailbox on disk. But that applies only if Postfix would be
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saving the email. In our setup the email will be passed on to Dovecot in a later step. Dovecot will then handle the
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files on disk. Postfix just has to know is whether a mailbox exists. And that's why any _answer_ is sufficient here.
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### Tell Postfix
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You have created the config files for the three mappings. Now you just need to tell Postfix to use them:
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```sh
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postconf virtual_mailbox_domains=mysql:/etc/postfix/mysql-virtual-mailbox-domains.cf
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postconf virtual_mailbox_maps=mysql:/etc/postfix/mysql-virtual-mailbox-maps.cf
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postconf virtual_alias_maps=mysql:/etc/postfix/mysql-virtual-alias-maps.cf
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chown root:postfix /etc/postfix/*.cf
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chmod o= /etc/postfix/*.cf
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```
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`postconf` is a command that changes configuration in `/etc/postfix/main.cf` and applies them instantly. You don't have
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to restart Postfix.
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The last two lines set the owner of the config files (_user=root, group=postfix_) and make sure that _others_ have no
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access. After all a database password is found in these files.
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### Test it
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Give the mappings a quick test using the `postmap -q` command (_q_ stands for _query_):
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```sh
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postmap -q example.org mysql:/etc/postfix/mysql-virtual-mailbox-domains.cf
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postmap -q jack@example.org mysql:/etc/postfix/mysql-virtual-alias-maps.cf
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postmap -q john@example.org mysql:/etc/postfix/mysql-virtual-mailbox-maps.cf
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```
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These three commands use the three mapping files (`*.cf`) to query your three database tables. The result should show:
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```
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1
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john@example.org
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1
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```
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If you get anything else, please check those three config files. Perhaps the database password is wrong?
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