Background
Users on a WHM/cPanel might encounter the following message:
"Domain example.com has exceeded the max defers and failures per hour"
The settings for this is controlled in two places in WHM and the are set to intelligent defaults to avoid users inadvertently spamming causing IP blacklisting.
To see the two places where this setting exist, go to:
1. Account / Package Settings
WHM>>List Accounts>>Modify An Account
Scroll down and look for “Maximum percentage of failed or deferred messages a domain may send per hour”
This is the global setting that applies to everyone.
2. Tweak Settings
WHM>>Server Configuration>>Tweak Settings
Check the Mail section for “Maximum percentage of failed or deferred messages a domain may send per hour”
Be sure that if you make changes, they are chosen extremely wisely and that you clearly document them.
Also note that changes to individual accounts have a priority over the global setting.
References
WHM forum post max defers and failures
https://forums.cpanel.net/threads/domain-has-exceeded-the-max-defers-and-failures-per-hour.587231/
https://forums.cpanel.net/threads/exceeded-the-max-defers-and-failures-per-hour-error.643813/