added example config for smtpd_sender_login_maps config

src/content/docs/ispmail-trixie/180-relaying.mdx

@@ -267,14 +267,31 @@ That will work:
 <~  250 2.0.0 Ok: queued as AFFF12292A
 ```
 
-<Aside type="tip" title="Allow aliases?">
+<Aside type="tip" title="Allow sending from an alias address?">
 
-The setup so far works for personal mails from John. But if John would be a member of a distribution list
+The setup so far works for personal mails from John. But if John was a member
-`pizza@example.org` then would not be able to use `pizza@example.org` as a sender address.
+of a distribution list `pizza@example.org` then he would not be able to use
+`pizza@example.org` as a sender address.
 
-If you want to allow that, create a new \*.cf file with a mapping query like this:
+If you want to allow that, add an SQL mapping for the `smtpd_sender_login_maps`
-`SELECT email FROM virtual_users WHERE email='%s' UNION SELECT destination FROM virtual_aliases WHERE source='%s'`.
+as follows:
 
+```sh title="Add an SQL mapping file"
+cat > /etc/postfix/mariadb-sender-login-maps.cf <<EOF
+user = mailserver
+password = MAILSERVER-PASSWORD-HERE
+hosts = 127.0.0.1
+dbname = mailserver
+query = SELECT email FROM virtual_users WHERE email='%s' UNION SELECT source FROM virtual_aliases WHERE destination='%s';
+EOF
+
+postconf smtpd_sender_login_maps=mysql:/etc/postfix/mariadb-sender-login-maps.cf
+```
+
+This SQL query will return…
+
+- the email address of the current user
+- all alias addresses that point to the current user
 </Aside>
 
 <Aside type="danger" title="Check your $mydestination">