How to really turn off already disabled AutoCorrect feature in Windows 10

I was going through the same issue and here is what I believe to be the most correct answer:

1. You cannot turn off autocorrect in Windows 10 mail. It lives its own life independant of Windows 10's "typing" settings.
2. You can, however, change the language for an ENTIRE mail, but it's not obvious how to do that - You might think selecting all and then choosing a language up in the language area at the top of the screen might allow you to type an email in ONE language, but you would be wrong for thinking that so stop it. (it's too logical for today's modern app design, you know)

4. Windows 10 Mail listens to the language you set in your system region settings! In order to type an entire mail in a single language, select your language down in Windows 10's language bar before typing. After every character, Windows 10 will poll the language set in the language bar and set it's own language back to that language. That's why setting a language in Mail itself isn't "sticky."

This is an old question but this might be the right answer.

The reasoning: Seems that group policies take precedency over user Settings.

What worked for me:

1. open the gpedit.msc (just type into the search/Cortana and click the first option)
2. go to User Configuration > Administrative Templates > Control Panel > Regional and Language Options
3. Enable both Turn off autocorrect... and Turn off highlight...

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I had the exact same issue so I switched to Thunderbird instead. But still many other apps used the spell checking and auto correct "feature" of Win 10 although it was turned off. But finally I found an easy and definite way to really turn it off in all apps:

1) go to C:\Windows\System32

2) rename MsSpellCheckingHost.exe to MsSpellCheckingHostOLD.exe (or any other name)

3) rename MsSpellCheckingFacility.dll to MsSpellCheckingFacilityOLD.dll (or any other name)

4) restart your PC

Works fine for me. No more spell checking, no more auto-correction. Please could somebody confirm this for the Mail-App as well since I completely removed the Mail and Calendar app from Windows.

Edit: in order to rename the files in system32 folder you have to gain ownership of the files:

rightclick - properties-> security-> advanced-> owner

now set permissions to read and write.