Like Raymond, my vote goes to Runbox. I have had an account with Runbox for well over two years and have been very impressed by their high level of support, stability and service (the three S's!). Runbox has an intuitive and highly visually appealing web interface which, as a Mac user, captured me the moment I saw it.
I acknowledge that both Fastmail and Mailsnare provide
excellent services but it is Runbox that I like best.
- Henry