This is a discussion on Best spell checker to review emails within the iPad OS forums, part of the Apple iPad Discussions category; Thanks!...
The only spell checker you can use in Mail is the system spell checker. You'll find the options under Settings > General > Keyboard. Spell checking and the options are improved under iOS 5. Under previous versions you couldn't turn off auto-correct without also turning off spell check.
It should be possible to to create your mail in another app then copy and paste it into an email. Most other apps that offer spell checking are probably going to use the same dictionary as the one in Mail. If there is one that uses it's own dictionary, I have not heard of it. Then again, I have not been looking for one either.