Unlike previous version, once the mail settings have been set up and saved, the password will revert to a default asterisk value. User then is unable to send a test message because the credential was "invalid".

Workaround to test email:
Input the mail configurations and immediately send a test mail without saving. Once the test succeeds, save the configuration

Another alternative way is to create a user and check "Email a link to the user to set their password" and see was the mail successfully sent.