If you receive an notification or error messages and you are unable to send or receive emails using Outlook;

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 this 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 this article on how to Modify mail server settings - Outlook

App Specific Passwords (ASP)

Have you recently setup 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 this article on how to Modify mail server settings - Outlook