I am running ZCS 3.1 on FC4. Web access to zimbra works fine, as does pop3 access via outlook. When using my laptop on our local network, SMTP also works correctly (i.e. I can send emails through the zimbra MTA using outlook). When outside the firewall, email that I send to domains not on our zimbra server are returned immediately with a 554 (relay access denied) error.

What do I need to change to allow SMTP relaying?