Thunderbird has no grammar checker or autocorrect. So I guess you will need to use the shift key.

There are a number of grammar type addons. One of them may offer the items you seek https://addons.thunderbird.net/en-US/thunderbird/search/?q=grammar&appver=&platform=

I always use the Shift key for this process. In fact, someone in 2015 was given the same answer by "Matt". I assume the questioner simply went to another program. I like Thunderbird but I'm becoming more and more annoyed by the lack of support (and I mean real support) for genuine problems. I'm an English teacher. I always use upper-case/capital letters at the start of my sentences because sentences start with capital letters. Thunderbird doesn't do this. It should. An add-on is not the answer. I'll rephrase my question. Why does Thunderbird change the first letter - when the user uses the Shift key to make it upper-case - back to lower-case? Because Thunderbird does this, what's the best way to train it to use and even accept capital letters at the beginning of sentences?

It answer your rephrased question: It doesn't, or never has when I've used it.

As for 'training' it, you can't.

It is not Thunderbird's responsibility to enforce writing rules. To many, email is a means of easy communication, not a formal document. For example, Thunderbird allows all upper-case paragraphs and doesn't require periods. An addon is the perfect solution as it is there for those who want it. I have no idea what you mean by a lack of support for genuine problems.

Before I look for another, more helpful email program, I'll attempt to educate those who are not interested in the dynamics of why people use certain computer programs. In my case, I'm politically and socially against the big billionaire corporates simply making more money to increase their power over people and governments. I've been using computers from before PCs and even managed a mainframe node for a major Australian government body. I got lazy, however, when Microsoft started producing programs that made it easier for everyone to submit to artificial intelligence. I'm also someone who appreciates the nuances of language in different cultures and even how words we use came from other languages and cultures. Some very lazy or exclusive people may use written communication loosely, to encrypt and encode - to appeal to those wrapped up in their club-like comfort zones. I, however, have found that people all over the world would prefer to be taught or to learn how to speak and write in language that follows certain formal patterns and that includes all who want to learn different languages. I'm not saying that loose forms of communicating in new and diverse ways shouldn't be allowed. I like playing with word patterns and rap and poetry with no full-stops, commas and capital letters... but not as a serious scholar or even journalist. Don't bother to answer with any more patronising or paternalistic 'suggestions'. I'm finding another shareware internet program and email program with a bit of wisdom and maturity behind them.

I was merely sharing some facts. I will save you some time in your pursuit of another email client: Becky!, Postbox, TheBat!, and Claws_mail do not enforce punctuation in any way, but I am unaware of Outlook, Mailbird, and eM Client. Since Outlook focuses strongly on the business community, it may offer the features you seek.

Before I look for another, more helpful email program, I'll attempt to educate those who are not interested in the dynamics of why people use certain computer programs.

If you have feedback, this is most certainly not the place for it. This is a peer support forum. Sure i gave essentially the same answer almost a decade ago, nothing has changed. You appear to think that is a failure. It well might be. But our opinions have no more weight than yours in the development process.

Recent versions of Thunderbird have a feedback entry on the help menu and it does not link to this support forum, instead directing you to https://connect.mozilla.org If you feel strongly enough about your ideas on the subject, I suggest you offer your feedback in a place someone other than your peers will read it. Personally I use the AI Grammar Checker & Paraphraser as my grammar is not good. However it is not what you asked for as it does not do autocorrect for leading upper case or common misspellings.