PS - Should have also said that my Windows 10 is the Home edition.

Good question. I would say the answer is MSWord has been designed as a complicated and extensive word processing piece of software that covers all functionalities specifically associated with word processing. It is expensive to produce and therefore requires payment for software. Most people use a fraction of it's capabilities.

As Thunderbird is a simple text email client with basic functions required for email purposes. It does not have the man power nor the Income like Microsoft and relies on donations and volunteers to assist with addons, bugs etc. Hence you get the option to flag potential spelling mistakes which you would need to correct if required.

Many thanks for your reply, Toad-Hall!

Yes, sadly, I guess that's it: as you say, MS Word is a wonderfully "extensive word processing piece of software that covers all functionalities specifically associated with word processing." Whereas, I'm realizing, that Thunderbird, while having some very good features (functionalities?), probably cannot ever compete there - particularly not with things (software?) such as Autocorrect.

I suppose one should try and remember to do all one's writing in a Word doc and then copy this into an email text field. Until such time, maybe, as when a text email client can come up with software capeable of doing Autocorrect.....

(Excuse me if I'm using inaccurate terminology for things like 'functionality', 'software' and others).

As a heads up....If you copy paste text from another software eg: MSWord, I would advise you use the 'Edit' > 'Paste without Formatting' option (Ctrl+Shift+V) otherwise you will end up inadvertantly copy pasting a load of extraneous code which will bloat the email and may cause readability or layout issues as most MS specific code only works properly in MS products.

Thanks again Toad-Hall for this useful advice! Yes, I'll try and remember to select the 'Paste without formatting' - a really useful feature/functionality.

I've seen the effects when I recieve emails and seen the odd signs where something's clearly not readable or supported on my screen, and I sometimes worry about what might come up on a recipient's screen as it seems like there's loads of incompatibilities around (Sorry, hope this makes sense!).

Just for another opinion, autocorrect is a two-edged sword. I have that on my phone and I have to watch it carefully for sometimes destroying my intended word. An addon that may interest you is the Grammar & SpellChecker one. It changes nothing, but it does a good job of prompting for sentence structure and common problems.