I just had a case where the message bounced with this
singularly unhelpful message:

> > If you do so, please include this problem report. You can delete your own
> > text from the attached returned message.
> >
> > The mail system
> >
> > <someotheruser@someotherdomain.com>: someotherdomain.com

with the relevant "details":

> > Final-Recipient: rfc822; someotheruser@someotherdomain.com
> > Original-Recipient: rfc822;someotheruser@someotherdomain.com
> > Action: failed
> > Status: 5.0.0
> > Diagnostic-Code: X-Postfix; someotherdomain.com

The corresponding log entries were no more helpful:

> > Feb 1 13:26:47 mta01 postfix/smtpd[25137]: warning: 1.1.1.1: address not listed for hostname somehost.domain.com
> > Feb 1 13:26:47 mta01 postfix/smtpd[25137]: connect from unknown[1.1.1.1]
> > Feb 1 13:26:51 mta01 postfix/smtpd[25137]: 3BBBC49137C: client=unknown[1.1.1.1], sasl_method=LOGIN, sasl_username=someuser@domain.com
> > Feb 1 13:26:51 mta01 postfix/cleanup[24210]: 3BBBC49137C: message-id=<00de01cbc256$bcbcd8e0$36368aa0$@com>
> > Feb 1 13:26:51 mta01 postfix/qmgr[20498]: 3BBBC49137C: from=<someuser@domain.com>, size=3089, nrcpt=1 (queue active)
> > Feb 1 13:26:51 mta01 postfix/smtpd[25137]: disconnect from unknown[1.1.1.1]
> > Feb 1 13:26:51 mta01 postfix/error[25284]: 3BBBC49137C: to=<someotheruser@anotherdomain.com>, relay=none, delay=0.05, delays=0.03/0.01/0/0.01, dsn=5.0.0, status=bounced (someotherdomain.com)
> > Feb 1 13:26:51 mta01 postfix/bounce[25286]: 3BBBC49137C: sender non-delivery notification: 46E7C49183B
> > Feb 1 13:26:51 mta01 postfix/qmgr[20498]: 3BBBC49137C: removed

I brought this up on postfix-users and was pointed at main.cf, where zimbra has:

virtual_transport = error

instead of:

virtual_transport = error:5.1.1 Local mailbox unavailable

while that's only a marginal improvement, at least there's an error message and it doesn't look truncated. I'm rusty on my postfix configs, but apparently this is a generated error rather than an error message override --- perhaps "User unknown" is the right error here?