When attempting to send a test email from the Mail Service configuration, the following error message is thrown, 

5.7.0 Too Many Unauthenticated commands

When accessing a Mail Service configuration, if you immediately try to send a test email, it will fail because the password has not been entered. The password textbox will be empty, which makes it necessary to enter it again before sending a test email.
Once the password has been entered, select Save then you should be able to send the test email.

KBA is applicable to both Cloud and On-Premise systems.