Doesn’t work – “Authentication Failure”

I’ve used this plugin on other websites with success. My most recent installation failed to authenticate. At first I thought it was an issue with the recipient mail server. However, when I tested the identical settings using another SMTP plugin, Easy WP SMTP, it worked without error. So the authentication failure clearly exists in WP Mail SMTP. Now I have to go back and check my other sites that are using this plugin.