Have you recently changed your password? 

If so, it could be that you have not yet updated the password in Outlook. Note, the password must be setup for both incoming and outgoing settings. 

Please refer to the article on how to Modify mail server settings - Outlook

Mail settings

There may be an issue or misconfiguration of the mail server settings. 

Please refer to the article on how to Modify mail server settings - Outlook

Have you recently setup App Specific Passwords (ASP)

If so, you will need to update the password in Outlook using the ASP. Note, the password must be setup for both incoming and outgoing settings. 

Please refer to the article on how to Modify mail server settings - Outlook

Other Outlook support issue

If you require additional support for Outlook, we recommend reviewing the support resources provided directly from Microsoft:  https://support.microsoft.com/en-au/outlook