From time to time I receive copies of my server’s outgoing bounce messages (from mailerdaemon) with the offending incoming messages attached.  The reason stated for the bounce, in this case, is that there is a non-existent domain in the From: header of the incoming message.  However that is not true; the domain name txercoupemuseum.org does exist:

/usr/bin/host -t a txercoupemuseum.org
txercoupemuseum.org has address 65.254.227.240

The log shows:

17:40:04.000 1 DEQUEUER [2420011] SYSTEM(gib) failed: non-existent domain (txercoupemuseum.org) in From:
17:40:04.802 2 SMTP-000007(*) [2420014] sent encrypted [192.168.1.2]:50534 -> [97.78.41.43]:2501, 
    got:250 4123891 message accepted for delivery

Do I have a DNS problem?  It happens too rarely to justify increasing the log level, but…?  Cheers,
--
Gib Henry