This is something that has annoyed me in Thunderbird for a long time. There's no autocorrect when you are typing mail. Outlook has had this for years, and it's something that you come to rely on when banging out email.
Well, I finally put my googling brain to the task and came up trumps. A Thunderbird extension.
You can find it here.
Now if only blogspot would implement it here.
[Update 24/2/10] - note the extension in the link above does not work in Thunderbird 3.x, only 2.x
[Update 11/3/10] - I've downloaded Phrase Express. That works, and works for all applications as a keyboard hook! Great stuff.
[Update 5/1/13] - Why are you still using a mail client at all? Its all in the cloud!