<p>Open / forward the following <code>TCP</code> ports to your mailserver's ip-address:</p> <pre><code>TCP / 25 (SMTP - server to server) TCP / 587 (SMTP - client to server) TCP / 993 (IMAP - client to server) TCP / 4190 (MANAGESIEVE - client to server)</code></pre> <!-- !!! EXPLAIN WHY --> <!-- REFERENCES -->