Is anyone familiar with an open-source mail blaster program? I am looking for a solution for my enterprise that will allow users to sign up for 1 of 3 e-mail lists autmatically (with auto-unsubscribe features built in) that will allow me to push out monthly newsletters. Would mailman be the way to go, or does someone have experience with other solutions?

The key is easy to use for my end users and my list administrators...