Nothing wrong with the creation of the mailman list, it's a mandatory action.
If you created a list Domain called list.example.com,here is what mailman does: In case you created a list called test for the domain mydomain.com it will create an alias
test@mydomain.com who points to
mydomain.com.test@lists.example.com. For the user of the list will still be
test@mydomain.com.
the list should be listed in /etc/postfix/virtusertable