5.3.3 Unrecognized command

I reviewed this support question. I am set up to use port 587 with STARTTLS and authentication, but am receiving this message:

You getting this message because an error detected while delivered your email.
For the domain: https://mydomain.com
The log to paste when you open a support issue:
5.3.3 Unrecognized command

Total Control Panel Login
To: [email protected]
From: [email protected]
You received this message because the domain phoenixtechnology.us is on the enterprise allow list. Please contact your administrator to block messages from the domain myemailprovider.us

Thank you

The page I need help with: [log in to see the link]