Unable to receive email - "5.3.0 - Other mail system problem 554-'5.2.3 MailPolicy violation Error delivering to mailboxes'"

Unable to receive email - "5.3.0 - Other mail system problem 554-'5.2.3 MailPolicy violation Error delivering to mailboxes'"

My users are unable to receive emails from one particular domain, apparently. The domain known to be kicked back is whitelisted in the spam control.

I sent an email to support earlier this morning but I have not received a reply.  The error in the title is what the external recipients receive.

Does anyone have any ideas as to what the cause is?  Per the email admin from the sender, the issue is on the Zoho side.

Thanks in advance for any help you guys can provide.