Why use Qpopper? Why not just use either Cyrus IMAP or Courier IMAP, for instance, which both support both IMAP and POP?
Cyrus IMAP is built into OS X Server and is well suited for building IMAP clusters (which you can do either with unified or standalone IMAP and Murder servers), is useful for designing workflows which involve automatic mailbox creation and deletion since the administrative API is an open Perl based API. Cyrus mailboxes are formatted so that each message is its own file, and there is also a database index.
If you are looking for something simpler, Courier IMAP uses the Maildir mailbox format, works with Cyrus SASL as an authentication layer that supports several authentication sources, and is generally easy to get going. There is also Dovecot which is a popular IMAP server which I've been meaning to look into.
Let us know if IDLE/Push support is important for you too...