If you take a look at Blitzmail you'll find that it actually hits most of your specs. It's still in use at Dartmouth (where I'm a student) but hasn't seen any major updates since the early 90s. It uses its own protocol instead of IMAP, which means you need to run the Blitzmail server too, but the server software supports Blitzmail, IMAP and POP. It also allows fuzzy matching of names when you send an email to someone in the Dartmouth directory, which is a pretty handy feature, also implemented on the server side. Spam filtering is done with Spam Assassin on the server side. It's extremely simple, but I'm going to guess that at least 80% of the people here still use it and swear by it. I've tried to convert to Thunderbird, but end up coming back to Blitz every time. Since it is basically stuck in 1994 it runs very quickly on any computer you can find. I can walk up to a public terminal, sign on, see that I have no new messages, and sign off in about 4 seconds. Anyway, if you're on vacation and have nothing better to do the technical details are here. While it's a little dated, it's proof that not all email has to look like Outlook and IMAP.