Cyrus

Cyrus IMAPd is an amazingly powerful IMAP/POP3 server that handles huge piles of mail without any great hassle at my work. I've been using it for years without complaint or issue.

It maintains a header cache for fast IMAP header downloads. It can do full-text indexing and server-side IMAP search with squatter. It supports sieve (obviously) and does so well. Mail is delivered into the system with LMTP.

You might want to look into it if you're dealing with truly huge volumes of mail. The server-side search in particular is a life-saver when dealing with big list archives.