[Exim] Advise on pop3 and imap servers`

I have been using exim here for some time now, and just now got around to
upgrading mine from version 3.x to 4.x. Since I host multiple domains with
my server, I decided to go ahead and implement support in exim for virtual
domains. The Exim part went flawless! Now what am I going to use to let my
users access their new virtual mailboxes?

Please pass on your recommendations for a pop3 and imap server that will
accomplish this. (I see a lot of options out there, but all I have found so
far don't seem to be under active development (which worries me).

Finally, I have seen several places talking about maildir. What is all the
hype about using maildir? I understand what it is and does, but I don't
understand it's advantages.

Thank You

Terry Shows
CSS
USA