Using the New Outlook for Mac features in business environments
EDIT: After posting this, I noticed the comments on the answer from Michelle-N that other users have come to the same conclusion as me. I am leaving this response because it contains some more detail on the specific steps to take to resolve the autocorrect issue.
First off, let me say that all the claims that the autocorrect is a Mac OS issue are just a distraction from the real issue which is a bug in the Outlook program. I went through all the recommended steps for disabling various settings in Outlook and the Mac OS, and nothing worked, except for one thing:
Go to Outlook > Settings > Spelling & Typing
The setting called "Correct Spelling Automatically". doesn't do anything. If you want the autocorrect to be turned off, you have to uncheck "Check Spelling and Grammar as I Type". It looks like that is the setting that actually applies autocorrect. Don't forget to restart Outlook after making the change.
This works for me on Microsoft Outlook for Mac Version 16.110.3 (26070318)