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 firstname.lastname@example.org
who points to email@example.com
. For the user of the list will still be firstname.lastname@example.org
the list should be listed in /etc/postfix/virtusertable