[Mailman-Users] Getting implicit destinattion even though To: field is correct

[Aaron Smith]

                I just installed mailman 2.1.20 using the Ubuntu provided package for 16.04.
I'm setting this up as a replacement for an older mailman server.  Right now, the lists are just using the machine name for the addresses, but whenever I send a message to a list address, it gets held for moderator approval, claiming it has an "implicit destination".  The server name is (not really) mail01.example.com.  mm_cfg.py has DEFAULT_EMAIL_HOST of "mail01.example.com", and I set up the necessary configuration to have the aliases generated (MTA=POSTFIX) and commented out the postfix_to_mailman.ph transport line in postfix.  However, if I send a message to testlist at mail01.example.com<mailto:testlist at mail01.example.com>, it is moderated for implicit destination.  Every single time.  And I KNOW that the To: field has the correct email address because I'm typing it in myself during a telnet session to port 25!  What am I  missing here?