If your having trouble diagnosing email issues with Zimbra Opensource edition, such as having trouble receiving external email look at the following wiki pages:

• http://wiki.zimbra.com/index.php?title=ZimbraMtaMyNetworks

• http://wiki.zimbra.com/index.php?title=Incoming_Mail_Problems

• http://wiki.zimbra.com/index.php?title=Administration_Console

If your mail is arriving at your server, but is being deferred, you may require split DNS if your behind a firewall or what works in some cases, add an alias with the external IP with the netmask of 255.255.255.255 if your are forwarding ports.