Outlook 2016 for macOS, Authentication Failure, O365

Intended Audience

People experiencing Authentication Failure with Outlook 2016 for macOS and O365.


  • Outlook 2016 for macOS stopped being able to connect to O365 and check mail, no error displayed at this stage.
  • Credentials are definitely correct, still able to access O365 mail via iOS app and browser.
  • Deleting and re-adding the account within Outlook now throws up "Authentication Failure".
  • Uninstalling and re-downloading Office 2016 for macOS made no difference.


Thank You Larry: Original solution article


Seems that the issue is caused by corruption of KeyChain entries related to Outlook.

  1. Go to KeyChain Access
  2. Search "Exchange" under Login --> All Items and delete everything
  3. Search "Office" and delete everything
  4. Search "ADAL" and delete everything
  5. Launch Outlook, try again