I'm assuming that you mean to check your spelling.

Menu: Tools: Options: Composition: Spelling

No, I mean auto-correct, the ability to type quickly and let the word correct itself, just as you can do with texting.

There is not that ability in Thunderbird, and a quick search of add-ons doesn't find anything for this either.

I believe that Autocorrect exists on mobile phones because it is more difficult to type on a mobile phone, thus more mistakes, than on a computer keyboard.

The solution is to learn how to type on a computer keyboard. When I was in school, learning to type on an old manual typewriter was mandatory. It is much easier to type on a computer keyboard. Do they not teach that in school?

You must be a glutton for punishment: http://www.damnyouautocorrect.com/

Bruce, Thanks for the prompt reply. I too learned to type on a manual typewriter and believe it or not, they required everyone at Drexel University (Class of '75) to take THREE, one credit typing classes during each Freshman trimester. Didn't stick then, ain't sticking now.

There is an add-on but it isn't actively maintained and in my experience, it is more trouble than it is worth. You cannot teach or train it and it persists in re-applying corrections regardless of how many times you tweak them. Its simplistic notions of words and grammar mean that it won't allow you to add some really useful terms to its "dictionary", such as postcodes. Abbreviations such as e.g. and i.e. will be a permanent cause of headaches.

https://addons.mozilla.org/en-us/thunderbird/addon/mms-auto-correct/

The user reviews suggest that it stopped working (again!) with TB45. :-(