Capitalization?!

No, there's not. Thunderbird includes a spell checker, but not a grammar checker or auto-correct feature.

Not the same thing, but you can add those words to the dictionary.

Thank you. Shame,but dictionary may be a good workaround. Is it easy to add to dictionary? Thanks again

Sorry I think I misunderstood. IS there any benefit to adding to dictionary? If it still woni't auto correct, or will this cause that behaviour to kick in? Thanks

The only thing adding it to the dictionary will do for you is to prevent the word from being flagged as an error. Probably not too useful.

Yeah I realised that! Does anyone know if either of these addons might help? (I have never installed addons before!)

https://addons.thunderbird.net/en-US/thunderbird/addon/auto-capitalize-mail/

https://reviewers.addons.thunderbird.net/EN-us/thunderbird/addon/freecorrector/

The first one is classified as "experimental" and there are no reviews, even after a year and a half. And no information about what, exactly, it does.

The second one also has no reviews, so there's no feedback on how well it works or not. It looks to me like you still have to click on a word to correct it - it doesn't do it automatically. But it's open source and the code is available and being actively worked on, which is comforting.

I'd tend to avoid the first and try out the second, though it may not quite do what you're looking for. It would be easy enough to remove if you don't like it.