How to fix "Domain xxx has exceeded the max defers and failures per hour (10/10 (100%)) allowed"?

For a period of time last night, all emails sent to one of my domains (hosted with shared hosting) were suddenly not delivered - including one from PayPal and a couple subsequent test emails I sent from GMail. Previously, I was receiving emails sent to my domain (all emails *@ are forwarded to a catch-all email) normally. I made no changes to the CPanel configuration recently.

In CPanel, info for the bounced email from PayPal gives the following errors: Domain <domain URL> has exceeded the max defers and failures per hour (10/10 (100%)) allowed. Message discarded.

Event: rejected rejected User: -remote- Domain: Sender: service@paypal.com Sent Time: Oct 6, 2014 7:13:19 PM Sender Host: mx2.slc.paypal.com Sender IP: x.x.x.x Authentication: unauthorized Spam Score: Recipient: <myuser>@<domain URL> Delivery User: <my CPanel login> Delivery Domain: <domain URL> Delivered To: Router: reject Transport: **rejected** Out Time: Oct 6, 2014 7:13:19 PM ID: 1XbKHS-0009E9-8O Delivery Host: mx2.slc.paypal.com Delivery IP: x.x.x.x Size: 0 bytes Result: Domain <domain URL> has exceeded the max defers and failures per hour (10/10 (100%)) allowed. Message discarded.

The error (per a test message that bounced back to GMail) is as follows:

`Delivery to the following recipient failed permanently:

@

Technical details of permanent failure: Google tried to deliver your message, but it was rejected by the server for the recipient domain by . [<y.y.y.y>].

The error that the other server returned was: 550-Domain has exceeded the max defers and failures per hour 550 (10/10 (100%)) allowed. Message discarded. `

I opened a ticket with the host with the above info and received the following extremely unhelpful response just under 7 hours later: Yes, your domain <domain URL> was exceeded the max defers and failures per hour allowed.

I'm having trouble finding the actual reason for emails to be bounced/rejected, much less figuring out how to fix it. This is a problem because it's a domain I use quite heavily for receiving email (though I only very rarely send emails from the domain address and I have configured GMail to use the GMail servers while doing so). Help, please?

ETA: I don't know why the markdown code I used to box the bounced emailed isn't working

http://ift.tt/1dvHQ0Z