Here is the setup:
both on the same Zimbra server. We have a Barracuda Spam filter as our Relay server for both incoming and outgoing email. I've got both the "Relay MTA for external delivery" and "Inbound SMTP host name" set to the IP of the Barracuda. The Barracuda forwards email either direction no problem.
I've got a user that wants aliases from domain2.com to forward to his domain1.com email address.
email@example.com -> firstname.lastname@example.org
email@example.com -> firstname.lastname@example.org
When an email is sent to either 2nddomain.com addresses it goes through a mail loop then gets a top many hops bounce sent back to the sender. I can send locally to email@example.com no problem. I don't know why, I would think Zimbra would see that 2nddomain.com is a local domain and not forward it out to the external relay?