If you receive an notification or error messages and you are unable to send or receive emails using mac Mail.

Have you recently changed your password? 

If so, it could be that you have not yet updated the password in Mail. Note, the password must be setup for both incoming and outgoing settings. Please refer to this article on how to Modify mail server settings - Mail (macOS)

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 - Mail (macOS)

App Specific Passwords (ASP)

Have you recently setup ASP?  If so, you will need to update the password in Mail 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 - Mail (macOS)