Is this the full error? If there is more information it may guide you further. You can check the qie logs for more detailed error as well:
https://www.qvera.com/kb/index.php/5/where-are-the-log-files-for-qie?show=5#q5
The AuthenticationFailedException tells me the server could not authenticate you.
Couple of things to check:
1. Username and password are correct.
2. Are you using TLS when you shouldn't or not using TLS when you should (Some servers require TLS and give authentication errors if not used.)
3. Is the QIE server authorized on the SMTP server (Some servers only allow certain IP address or range to relay on the SMTP port)
4. Make sure SMTP is allowed (Gmail and other services make you update the preferences to allow SMTP relay. Here is a website on how to allow SMTP in Gmail: https://support.google.com/a/answer/2956491?hl=en).
