How to Fix and/or Disable Auto-Complete in the To: address field in M365 Outlook (Windows)
1. Open the M365 Outlook Client
2. Click on File
3. Click on Options on the left-side navigation pane
4. Click on the Mail tab, then click on Empty Auto-Complete List, then click Yes in the pop up confirmation, then click OK
Send a few test emails to make sure Auto-Complete is now working properly, if not you will have to disable Auto-Complete by following the directions below.
1. Click on File
2. Click on Options on the left-side navigation pane
3. Click on the Mail tab, then make sure the Use Auto-Complete List to suggest names… option is unchecked, then click OK