I use Evolution. It has a few features I haven't found in other e-mail clients:
* Ability to specify only a part of the message as "preformatted text", so they don't get word-wrapped. Useful for pasting console output in support messages, for example.
* Nice "bullet list"/"number list" support.
* Pretty good IMAP support.
* Multiple-language spell-checking in a single mail.
It's bloated and slow, but newer versions actually do fix lots of bugs.