Turn off Autocorrect in Apple Mail

If you believe the Auto Correct ON or OFF should apply globally in the Mail Application in its entirely

Then have a few suggestions ;

1 - Get Support Choose a product and we’ll find you the best solution.Start now and open an Apple Support Ticket as they are Apple Employees to deal will these types of issues . 

2 - Product Feedback - Apple and make it known to Apple regarding this ongoing issue  or Feature Addition

FYI at this point in the Development of Ventura. It is doubtful any new Features will be added to this version of macOS.

As Apple is working on the Final Release Version of Sonoma macOS 14 and, no doubt , will be working on subsequent updates to macOS 14

You can refer to this official apple sultion to fix this Check spelling and grammar on Mac - Apple Support

if you still same you can reach out to apple official support to get quick assistance Official Apple Support

Give this a try: boot into Safe Mode according to How to use safe mode on your Mac and test to see if the problem persists. Reboot normally and test again.

NOTE 1: Safe Mode boot can take up to 3 - 5 minutes as it's doing the following; 

• Verifies your startup disk and attempts to repair directory issues, if needed

• Loads only required kernel extensions (prevents 3rd party kernel/extensions from loading)

• Prevents Startup Items and Login Items from opening automatically

• Disables user-installed fonts 

• Deletes font caches, kernel cache, and other system cache files

NOTE 2: if you have a wireless keyboard with rechargeable batteries connect it with its charging cable before booting into Safe Mode. This makes it act as a wired keyboard as will insure a successful boot into Safe Mode.

Have you installed and run any "cleaning", "optimizing", "speed-up", anti-virus or VPN apps on your Mac?

You can't turn it off for just Mail. You'll have to turn it off for everything in the System settings:

Mail >> Main Tool Bar at Top

Edit >> Down Menu

Refer to image below

Thanks. Learn something new here every day.

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Autocorrect works for me in Mail and all apps.