Every once in a while, the Zimbra admin mail report will have a few errors, like last nights:

Code:
Errors
2009-04-08 12:19:51 bounced (User unknown in virtual alias table)
2009-04-08 12:19:51 bounced (User unknown in virtual alias table)
2009-04-08 11:42:15 bounced (User unknown in virtual alias table)
2009-04-08 12:19:51 bounced (User unknown in virtual alias table)
2009-04-08 11:42:15 bounced (User unknown in virtual alias table)
2009-04-08 11:42:15 bounced (User unknown in virtual alias table)
2009-04-08 11:42:15 bounced (User unknown in virtual alias table)
2009-04-08 11:42:15 bounced (User unknown in virtual alias table)
2009-04-08 12:19:52 bounced (User unknown in virtual alias table)
2009-04-08 12:19:52 bounced (User unknown in virtual alias table)
2009-04-08 11:42:16 bounced (User unknown in virtual alias table)
2009-04-08 12:19:51 bounced (User unknown in virtual alias table)
This really isn't all that helpful, because then I have to go to the system itself and grep for that message in /opt/zimbra/log and /var/log. But that's a grip for a different thread.

I found the source for these cryptic errors in /var/log/mail.log:

Code:
Apr  8 11:42:15 zimbrahost postfix/error[18337]: AE8071DC19F: to=<admin@zimbrahost.domain.tld>, relay=none, delay=0.1, delays=0/0.05/0/0.05, dsn=5.0.0, status=bounced (User unknown in virtual alias table)
Apr  8 11:42:15 zimbrahost postfix/error[18341]: C0C141DC1C0: to=<admin@zimbrahost.domain.tld>, relay=none, delay=0.03, delays=0.01/0.01/0/0.02, dsn=5.0.0, status=bounced (User unknown in virtual alias table)
Apr  8 11:42:15 zimbrahost postfix/error[18337]: CEB721DC1C0: to=<admin@zimbrahost.domain.tld>, relay=none, delay=0.02, delays=0.01/0.01/0/0.01, dsn=5.0.0, status=bounced (User unknown in virtual alias table)
Apr  8 11:42:15 zimbrahost postfix/error[18341]: EC4901DC1F2: to=<admin@zimbrahost.domain.tld>, relay=none, delay=0.01, delays=0.01/0/0/0, dsn=5.0.0, status=bounced (User unknown in virtual alias table)
Apr  8 11:42:15 zimbrahost postfix/error[18337]: EE5061DC1F6: to=<admin@zimbrahost.domain.tld>, relay=none, delay=0.02, delays=0.02/0/0/0, dsn=5.0.0, status=bounced (User unknown in virtual alias table)
Apr  8 11:42:16 zimbrahost postfix/error[18341]: 119281DC1C9: to=<admin@zimbrahost.domain.tld>, relay=none, delay=0.01, delays=0.01/0/0/0, dsn=5.0.0, status=bounced (User unknown in virtual alias table)
Apr  8 12:19:52 zimbrahost postfix/error[3848]: BBC701DC1FE: to=<admin@zimbrahost.domain.tld>, relay=none, delay=1.1, delays=0.15/0.56/0/0.37, dsn=5.0.0, status=bounced (User unknown in virtual alias table)
Apr  8 12:19:52 zimbrahost postfix/error[3860]: E4E271DC1C6: to=<admin@zimbrahost.domain.tld>, relay=none, delay=0.91, delays=0.13/0.43/0/0.36, dsn=5.0.0, status=bounced (User unknown in virtual alias table)
Apr  8 12:19:52 zimbrahost postfix/error[3857]: BBF171DC202: to=<admin@zimbrahost.domain.tld>, relay=none, delay=1.1, delays=0.15/0.56/0/0.37, dsn=5.0.0, status=bounced (User unknown in virtual alias table)
Apr  8 12:19:52 zimbrahost postfix/error[3859]: BC7B81DC204: to=<admin@zimbrahost.domain.tld>, relay=none, delay=1.1, delays=0.15/0.57/0/0.36, dsn=5.0.0, status=bounced (User unknown in virtual alias table)
Apr  8 12:19:52 zimbrahost postfix/error[3861]: 1410F1DC1D7: to=<admin@zimbrahost.domain.tld>, relay=none, delay=0.77, delays=0.15/0.26/0/0.36, dsn=5.0.0, status=bounced (User unknown in virtual alias table)
Apr  8 12:19:52 zimbrahost postfix/error[3848]: 3DB6F1DC1ED: to=<admin@zimbrahost.domain.tld>, relay=none, delay=0.61, delays=0.12/0.47/0/0.02, dsn=5.0.0, status=bounced (User unknown in virtual alias table)
Heart jumping into my throat and thinking there was a problem, I tried to replicate it by doing a manual SMTP connection via telnet from the local host to port 25, but everything works fine now.

What would cause the virtual alias table to temporarily lose an alias? So far, it's only seemed to hit the admin user, but why? How worried do I need to be that this'll drop mail for a real user?