Authentication Problem with Sending Email in Office365 using SMTP

If SMTP authentication (using login/password) fails for your smtp.office365.com account, then it may be that your account is configured to require MFA (multi-factor authentication).   You may need to change settings to allow for legacy authentication (single-factor auth).   See https://docs.microsoft.com/en-us/azure/active-directory/conditional-access/block-legacy-authentication

Also, an app password may be required.  See https://docs.microsoft.com/en-us/azure/active-directory/user-help/multi-factor-authentication-end-user-app-passwords