exim

Domain example.com has exceeded the max defers and failures per hour (10/10 (52%)) allowed. Message discarded.

I believe that I have fixed the problem on my own, but it may still be a bug with cPanel.

I found the following file:

/var/cpanel/email_send_limits/max_deferfail_example.com

Deleting this file fixed the problem immediately. I'm guessing that the system should have deleted (or updated?) this file at some point, but didn't.