For newer versions of Outlook
Run a self-diagnostics tool
You may be a Microsoft 365 customer who is experiencing the error that is described in the "Symptoms" section when you try to connect to an Exchange Online mailbox. You may also already be running newer versions of Outlook 2013 or Outlook 2016. If both conditions are true, you can run diagnostics to determine the issue that might cause the error. The diagnostics will perform automated checks and provide possible solutions to fix the detected issue.
Select Diag: Outlook keeps asking for my password to launch the diagnostic or click the link below