I got a zimbra e-mail server and I had this particular problem it took zimbra 5 days to notify the senders that the adress was invalid...
in zimbra.log after 40 hours :
Mar 13 07:56:35 mydomain postfix/smtp: 5A90834115: to=<firstname.lastname@example.org>, relay=none, delay=147372, delays=147369/0.02/2.4/0, dsn=4.4.3, status=deferred (Host or domain name not found. Name service error for name=invalidaddr.com type=MX: Host not found, try again)
the senders should be notify immediately if the e-mail is invalid.
in this particular case the e-mail was sent march 11 and my user received the error march 16.