You might check your /var/log/zimbra.log and/or mail.log files for more detailed information. I find that when I get those unhelpful error messages in a bounce, one of those two logs frequently has a whole bunch of useful detail.
Since the logs are pretty huge, you might try something like
where firstname.lastname@example.org is substituted with the actual email address that is the problem. If you find anything interesting let us know.
tail -1000 /var/log/zimbra.log | grep email@example.com