Our mail addresses are like firstname.lastname@example.org, but sometimes in received mail zimbra changes this to "email@example.com"@mta_server_name, where mta_server_name in the name of our mta server.
So person who receives mail see strangly looking mail address in "To:" field.
If I go to the zimbra mail server or to ironport and see logs everything looks OK and addresses in "to:" filed are fine.
If they were not we probably wouldn't even received this messages.
I gues this is a bug in zimbra's mta because this happens only from time to time. Can you confirm that this is a bug or is something wrong configured ?