[Mailman-Users] 550 Recipient Address Rejected Error

[Phelps Williams]

After creating a new list with a (clearly not properly setup) new install of
mailman, when sending a message to the list address I get the following
error back:

This is an automatically generated Delivery Status Notification

Delivery to the following recipient failed permanently:

        testlist at mydomain.com

Technical details of permanent failure:
PERM_FAILURE: SMTP Error (state 9): 550 <testlist at mydomain.com>: Recipient
address rejected: User unknown in local recipient table

I pulled my actual domain out of this error.  Oddly enough all messages
regarding subscribing etc work just fine.  Users simply cannot post
messages.  Any ideas?  Thanks.