 /var/log/maillog
mta traffic is logged to /var/log/maillog
To find these messages, run postqueue -p (as the zimbra user) to see if they're stuck in the mta queue. If they are, there should be some indication of why they haven't been deliverd.
If they aren't in the queue, check /opt/zimbra/log/zimbra.log to see what the lmtp server did with the messages when they were delivered to it... |