I was wondering if anyone could shed some light on this issue and probably point out something really obvious that I am missing completely.

I have just set up a Zimbra server for someone, based on a NAT setup. The router/firewall forwards all requests on port 25 to 192.168.0.170 where the Zimbra server sits with its own DNS server for DNS resolution. Incoming email works fine with incoming email accepted and delivered to the users inboxes. Unfortunately however the server cannot seem to deliver outgoing email at all. It looks up the MX records just fine but when it tries to connect to them it says the following in the deferred mail queue (The below example is for trying to send an email to aol.com)...

connect to aol.com[205.188.142.182]: connection timed out

There are no firewalls or anything that would block it from connecting and the router allows all outgoing connections from LAN IP's with no filtering whatsoever. Another mail server sitting on a different machine can relay email out no problem.

Is it possible this could be a problem with Zimbra somewhere that I am missing or is there something in my NAT setup I need to go back and look at? Any help would be fantastic. Many thanks.