I guess that it will not be easy for us to know what happened. It says that the authentication failed. That could be a wrong configuration in your mendix application but it may also be some security whatever of your mail server. Are you allowed to access it from any machine on the internet?
Pls check, it maybe the case that the two factor authentication is on.
And pls check that all the parameters are properly filled.
Don't forget to re-check the username and password
Hope it helps!