I'm looking to add an IMAP server on top of my current setup, so that I can try other mail clients but keep my existing setup for use over ssh connections. My current setup is to use fetchmail, which delivers to procmail, which sorts into mbox-format files in ~/mail. I use Mutt to read my email. I have exim for outgoing mail.

All the IMAP servers I've come across so far don't support mbox or only show /var/spool/mail/$user. The server would also have to be able to handle me changing the files manually in Mutt. Does anyone have any recommendations on a good IMAP server to look into?

I recommend Courier Imap.

As far as mbox, if you are serious about making your mail server the best it can be you might want to look into Maildirs. I could go into a million reasons why it blows mbox out of the water, but I don't want to waste your time. Maildirs are supported out of the box by Postfix and Qmail.