Update documentation about email configuration

This makes it explicit that outgoing traffic (25/587) needs
to be let through, as well as documenting how to debug
other non-delivery issues.

docs/configuring-playbook-email.md

@@ -19,3 +19,7 @@ matrix_mailer_relay_auth: true
 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).
+
+If you're having trouble with email not being delivered, it may be useful to inspect the mailer logs: `journalctl -f -u matrix-mailer`.