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# Postfix

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

Now that other mail servers can locate your mail server, the next step is to make sure it can receive and process incoming emails. This task is handled by Postfix, which communicates using the SMTP protocol.

Postfix knows if an email address is valid by checking for…

- Virtual Domains (am I responsible for this domain?)
- Virtual Aliases (do I have to redirect this address to another address?)
- Virtual Mailboxes (do I have a valid mailbox to store the email to?)

### Virtual Domains

Section titled “Virtual Domains”

There are different ways for Postfix to get that information. From text files, LDAP, MongoDB or PostgreSQL. Or via MariaDB – which is what we will use. But whatever kind of data source you configure, Postfix needs the information as a mapping. Consider it as a question and an answer.

- Question: is `example.org` one of our virtual domains?
- Answer: yes

If the information is not available you just get no answer:

- Question: is `example.net` one of our virtual domains?
- Answer: (no answer)

In SQL language Postfix has to ask MariaDB the question:

    SELECT "yes" FROM virtual_domains WHERE name='example.net';

Run the following code in your shell to create a configuration file creating that mapping:

Run this on your server

    cat > /etc/postfix/mariadb-virtual-mailbox-domains.cf << EOFuser = mailserverpassword = MAILSERVER-PASSWORD-HEREhosts = 127.0.0.1dbname = mailserverquery = SELECT 1 FROM virtual_domains WHERE name='%s'EOF

This has created a configuration file called `/etc/postfix/virtual-mailbox-domains.cf`. The `user`, `password`, `hosts` and `dbname` definitions tell Postfix how to connect to the database. And the `query` asks the question if a certain domain is present in the database table `virtual_domains`. Using `SELECT 1` we return just the number 1 if such an entry was found. In fact it does not matter what we return. Any answer is fine.

### Virtual Aliases

Section titled “Virtual Aliases”

Now let’s deal with aliases. As shown earlier an alias redirects an email to another email address. So the question and answer game goes like this:

- Question: is there an alias for `jack@example.org`?
- Answer: `john@example.org`

Postfix would then check if there are further aliases:

- Question: is there an alias for `john@example.org`?
- Answer: (no answer)

Apparently, there aren’t any more. So the email will go to `john@example.org`.

Run this code to create the appropriate mapping file:

Run this on your server

    cat > /etc/postfix/mariadb-virtual-alias-maps.cf << EOFuser = mailserverpassword = MAILSERVER-PASSWORD-HEREhosts = 127.0.0.1dbname = mailserverquery = SELECT destination FROM virtual_aliases WHERE source='%s'EOF

The query now gets all the `destination` email addresses from the database for a certain `source` email address.

### Virtual Mailboxes

Section titled “Virtual Mailboxes”

The last mapping we need is a query to find valid mailboxes. Without further ado:

Run this on your server

    cat > /etc/postfix/mariadb-virtual-mailbox-maps.cf << EOFuser = mailserverpassword = MAILSERVER-PASSWORD-HEREhosts = 127.0.0.1dbname = mailserverquery = SELECT 1 FROM virtual_users WHERE email='%s'EOF

Just the same story as with virtual domains. The database returns `1` if there is a `virtual_users` record matching that specific email address.

Actually the answer is usually not `1` but the path to the mailbox on disk. But that applies only if Postfix would be saving the email to disk on its own. In our setup the email will be passed on to Dovecot in a later step. Dovecot will then handle the files on disk. Postfix just has to know is whether a mailbox exists. And that’s why any answer is sufficient here.

### Tell Postfix

Section titled “Tell Postfix”

You have created the config files for the three mappings. Now you just need to tell Postfix to use them:

Run this on your server

    postconf virtual_mailbox_domains=mysql:/etc/postfix/mariadb-virtual-mailbox-domains.cfpostconf virtual_mailbox_maps=mysql:/etc/postfix/mariadb-virtual-mailbox-maps.cfpostconf virtual_alias_maps=mysql:/etc/postfix/mariadb-virtual-alias-maps.cf
    chown root:postfix /etc/postfix/mariadb-*.cfchmod o= /etc/postfix/mariadb-*.cf

`postconf` is a command that changes configuration in `/etc/postfix/main.cf` and applies them instantly. You don’t have to restart Postfix.

The last two lines set the owner of the config files (user=root, group=postfix) and make sure that others have no access. After all a database password is found in these files.

Test it

Give the mappings a quick test using the `postmap -q` command (q stands for query):

Run this on your server

    postmap -q example.org mysql:/etc/postfix/mariadb-virtual-mailbox-domains.cfpostmap -q jack@example.org mysql:/etc/postfix/mariadb-virtual-alias-maps.cfpostmap -q john@example.org mysql:/etc/postfix/mariadb-virtual-mailbox-maps.cf

These three commands use the three mapping files (`*.cf`) to query your three database tables. The result should show:

    1john@example.org1

If you get anything else, please check those three config files again.