Dovecot

I actually like Archiveopteryx, as well as all the other IMAP servers. I wish there were more of them.
It's boring when there isn't much competition.

Anyway, since Dovecot's scalability was mentioned, I'd just like to mention that Dovecot's upcoming
v2.0 adds support for a high-performance mailbox format called "mdbox"
(http://wiki.dovecot.org/MailboxFormat/dbox) that scales a lot better than Maildir. For typical users
it shouldn't be a problem to have something like a million messages in a mailbox.

Also if the problem is searching performance, Dovecot supports full text search indexing with Solr.