Update documentation about email configuration (relayhost brackets)

Relay hostnames that have MX records are looked up by postfix
and the MX record's payload is used instead.

This special behavior may be undesirable, so we make sure to
point it out.

docs/configuring-playbook-email.md

@@ -7,7 +7,15 @@ This may or may not work, depending on your domain configuration (SPF settings,
 
 By default, emails are sent from `matrix@<your-domain-name>` (as specified by the `matrix_mailer_sender_address` playbook variable).
 
-Furthmore, if you'd like to relay email through another SMTP server, feel free to redefine a few more playbook variables.
+
+## Firewall settings
+
+No matter whether you send email directly (the default) or you relay email through another host (see how below), you'll probably need to allow outgoing traffic for TCP ports 25/587 (depending on configuration).
+
+
+## Relaying email through another SMTP server
+
+If you'd like to relay email through another SMTP server, feel free to redefine a few playbook variables.
 Example:
 
 ```yaml
@@ -20,6 +28,9 @@ matrix_mailer_relay_auth_username: "another.sender@example.com"
 matrix_mailer_relay_auth_password: "some-password"
 ```
 
-**Note**: no matter whether you relay email through another host (by defining `matrix_mailer_relay_host_name`) or you let the local (in-container) postfix deliver directly, you'll probably need to allow outgoing traffic for TCP ports 25/587 (depending on configuration).
+Keep in mind that postfix will look up the MX record of your relay host (`matrix_mailer_relay_host_name`) and, if available, will actually use that instead of what you've defined. This behavior is [documented here](http://www.postfix.org/postconf.5.html#relayhost). If you'd like to suppress this and use the relay host value as is, wrap it in square brackets (e.g. `matrix_mailer_relay_host_name: "[mail.example.com]"`).
+
+
+## Troubleshooting
 
 If you're having trouble with email not being delivered, it may be useful to inspect the mailer logs: `journalctl -f -u matrix-mailer`.