I've followed the instructions in this thread without success. /opt/zimbra/log/convertd.log doesnt exist on my system.

The only solution I've found is to remove the mailbox in question, recreate it, then flush the queues. This is obviously less than optimal. It also seems that once there is an error like this concerning one mailbox all subsequent mail to that box results in the same error:

May 7 12:43:26 localhost postfix/lmtp[16765]: C241B84C43D: to=<xxx@yyy.com>, relay=zimbra.zzz.com.au[], delay=2350, status=deferred (host zimbra.zzz.com.au[] said: 451 4.0.0 Temporary message delivery failure try again (in reply to end of DATA command))

Any help would be appreciated.