Auto-correct seems to be enabled by default in some macOS computers which can be a pain especially in Mail, and any application which you are typing a lot and simply don’t need it. Here is how to turn off auto-correct for macOS applications.
Put a stop to It Globally
Go to the Apple Menu > System Preferences >
Click on Keyboard > Text and turn Autocorrect off by unchecking Correct spelling automatically
Turn off AutoCorrect per Application
Or if you see the benefits of it for most Applications and just want to disable one or two – then you can do this in the Applications Edit Menu under Spelling and Grammar and disable ‘Check Spelling While Typing’
This also works on older macOS versions down to OSX 10.9 Mavericks.