Thunderbird is alright but I think it's in need of what Firefox was to Mozilla Suite – a lean and mean cross platform IMAP client with a nice UI, and that's it.

Thunderbird is alright but I think it's in need of what Firefox was to Mozilla Suite – a lean and mean cross platform IMAP client with a nice UI, and that's it. I don't want an Outlook-style client that's full of things that aren't email (calendar etc).