I have been using Mailman for my users to communicate with each other through email. My hosting provider imposed a limit of 100 users for the discussion list so when user 101 came along I had a problem!
Mailster has fixed that and I can now have a discussion list with up to 1000 users! Also, it now has a subscription service for our newsletter.
Support was excellent as well with the couple of minor problems I had - and fast.