The daily admin emails for the Appliance indicate that there is a deferred message. /var/log/zimbra.log has this when I grep for deferred.

Aug 24 15:54:00 zimbra postfix/smtp[21884]: 5AB2D3411F: to=<>, relay=MAILRELAY:25, delay=4.7, delays=0.12/0.06/1.3/3.2, dsn=4.0.0, status=deferred (host MAILRELAY said: 451 <SENDERADDRESS>... Sender domain must resolve (in reply to MAIL FROM command))
Is there any way to flush the queue manually with the appliance?

A little searching turns up this thread ( which indicates that using the postsuper might work. Is this the best way to do this task?