1. Quit Outlook for Mac.
  2. In your Applications folder, find and open Keychain Access. It may be in the Utilities folder.
  3. On the left sidebar, under Keychains, select login.
  4. Under Category, select All Items.
  5. In the top right, search Exchange.
  6. Select all the results, right-click (or two-finger click), and select Delete (all) items.
  7. Repeat steps 5 and 6, searching for Office, making sure to delete all the results.
  8. Repeat steps 5 and 6, again, searching for ADAL, making sure to delete all the results.
  9. Launch Outlook for Mac.
  10. Enter your password in the prompt, and complete 2-factor authentication, if asked.