[Mailman-Users] 550 Recipient Address Rejected Error

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@mydomain.com

Technical details of permanent failure: PERM_FAILURE: SMTP Error (state 9): 550 <testlist@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.