Thanks pxl. That does get most of the way there. The thing that is missing is a single mail box. With the solution you suggest, users can send and receive email from the address, but there isn't a single place where all email is stored. I think what I am thinking of is actually only possible with microsoft exchange or with a more complex custom ldap system. The idea is a single mail account with it's own folders, but multiple users can log into the same imap accounts with different usernames. This allows you to keep email in a single place in a searchable form, kind of like a mailing list, but much easier to search.
I am pretty sure it isn't possible though, so unless anyone has any other ideas I think I will just let this one go.