While I have never used it, Mulberry is an Email client praised by many power users for its large set of features and standards compliance. The company that made it went bankrupt in 2005, but it's back again and released as open source. I should give it a test drive and write a review.
Sidenote: Mulberry is developed by a very talented Cyrus Daboo who now works at Apple on their CalDAV server.
