I have similar issues. I was getting this in Starter version, I bought Essential with some hopes. But I am getting the same following errors. I am not sure how to fix it - tries PHPmailer and SMTP both have failed:
This message was created automatically by mail delivery software.
A message that you sent could not be delivered to one or more of its
recipients. This is a permanent error. The following address(es) failed:
Domain amardeep.com.au has exceeded the max defers and failures per hour (5/5 (100%)) allowed. Message discarded.
---------- Forwarded message ----------
From: Amardeep Singh <firstname.lastname@example.org>
To: Amardeep Singh <email@example.com>
Date: Fri, 29 Nov 2019 05:59:26 +0000
Subject: Test e-mail from https://amardeep.com.au/
If you receive this message, that means your configuration is Ok
Not sure to fix it.