On a related topic -- and, again, tangentially related to email -- I'm using a program called BigSpeed Net (
www.bigspeed.net), which allows a POP3 mail account to be used as a P2P network. I use it as a way to transport large amounts of data (backup files, etc) from my home PC to my office PC, but could be used for any private P2P-type application among a group of users. Another pretty cool idea that makes use of the storage available on POP mail servers.