Maildir, unless I'm mistaken, is still a single file per mailbox.

Nope, with maildir, each message is a file, and each mailbox is three folders. cur, new and tmp. new is where mail is delivered, cur is where it's moved when a mail program touches it (not read, just touches it), and tmp is self explanitory. Theres several more rules to define it, like the filesystem date being the date it arrived, but that pretty much covers it.

in fact, I'll bet in your case it will take procmail's place, since you're probably using procmail as your MDA

Hmm, ok. My question is then, is there a really good spam filter for it? The only reason I am tied to procmail is the filter from www.spambouncer.org. It works really well, and saves me and my other users a ton of time. If I can replace it easially, I'll be happy to dump procmail, as I find it a pain for anything else. I was dreading the web interface I was thinking of creating for it.

Ok, so far you have convinced me to convert as long as you can answer two questions.

1. Can I install it alongside another IMAP server and change it's port for testing and such. I want to try it, but without bothering the existing setup. Plus, the existing setup will probably be phased out slowly, moving to a complete change when I reload the box on new hardware later this year.

2. What about archiving? I still want an easy way to do this, and working with a propritary mailbox format could complicate things.