Hi Bill.
No offense intended in 'responders'
We've managed to fix our specific problem, but I'm still struggling to understand the logic.
Our zimbra installation is behind a firewall.
The MX record for the domain is not the zimbra server but rather a spamfiltering device.
under global settings, MTA i entered the Inbound SMTP host name.
When "enable DNS lookups" was disabled zimbra would try to send email to the ip address of the A record for the target domains. ie gmail, hotmail etc.
Example: trying to email
xxxxx@gmail.com, zimbra will resolve the MX record to an ip address of: 74.125.127.100, while what it should have done is resolve it to 209.85.221.52 or one of the other 3 mx records.
if we look at outgoing email only for example, this isn't an expected behavior. at least not with other email suites I used before.
I resolved our specific issue, by setting up "false" dns entries in an internal DNS server (split dns?) that will allow zimbra to resolve the MX record for it's own domain to the local private ip, and then enabling DNS lookups under the MTA.
My biggest question though is: why would zimbra resolve the MX records incorrectly, when the DNS lookup under MTA was off?
Thx
Amir