To access Auto-Complete settings follow the steps below:

1. Open up your Outlook app.
2. Click on "File" menu in the top left then click "Options".
3. Select mail and scroll down to the "Send messages" section.

If you want Outlook to suggest and auto-complete names when typing in the To, Cc, and Bcc lines ensure "Use Auto-Complete List to suggest names when typing in the To, Cc, and Bcc lines" is checked.

Clearing and Regenerating your Auto-Complete Cache

If your email was recently migrated to Exchange Online you will need to do this process.

1. Open up your Outlook app.
2. Click on "File" menu in the top left then click "Options".
3. Select mail and scroll down to the "Send messages" section.
4. Select "Empty Auto-Complete List".

This will clear out the auto-complete cache allowing it to regenerate from Exchange Online.