GNU Mailman 3 is the name of the software that handles mailing list messages "Postorius" is the name of the web user interface software people use to manage their subscriptions

The idea of a single Postorious username/password for multiple mailing lists is nice idea, but we now know through painful experience how confusing it can be for end users if they find themselves with a mailman subscription that Postorious does not "know about". The extra step of creating a Postorious account to claim existing mailman subscription(s) is a lot to ask of long time users since it breaks decades of experience and expectations.

-Joe KM1P