I’m unable to send mail with the plugin nor am I able to use the other SMTP plugins for WordPress. (I get the error: Relaying not allowed with wordpress@XXX.com. That domain uses Zoho for mail hosting so this is very frustrating.) so 

I created a Sever-side app in Zoho's Developer Console. The Client ID and Secret I added to the plugin's fields are correct. I don’t have TFA turned on.

(But when I did, I created an app-specific password but there seems to be no field in the plugin to put the password.)

It’s not my webhost, Namecheap, because gmail SMTP works fine.