Mail Group
MailGroup is a simple application that does a lot. If you have many customers that need to receive emails from you, but you are limited by the number of messages that you can send at any given time, you want MailGroup. It allows you to send to virtually an unlimited number of recipients, giving each user an opt-in/opt-out option. It also allows you to house multiple mail groups at a time, with separate usernames for each one. You can also have multiple installs of the application running from the same database.
The best thing is the admin interface, which allows you to add/modify Terms of service, add moderators, and users, and shut down the system. If you shut it down, nobody can login (users), and those that are logged in are unable to send mail.

