If you are receiving an error similar to “Domain example.com has exceeded the max defers and failures per hour (5/5 (100%)) allowed“ in an email bounceback, it means that outgoing email from your domain has triggered a rule in the server that will stop any further email from going out of the server. This happens when a domain account sends out emails that either fail or get deferred. If the number of failures or defers are greater than the server limit, no further mails are allowed.
This error is derived from an hourly monitoring system, where any blocked domains are allowed to send email again at the top of every hour. For example, if you received this error at 5:30pm, the block will release at 6:00pm and the domain will be able to send emails out once more.
The solution is to wait and it will unblock itself but if the problem with email flooding exist it will trigger again the rule and block the sending for another hour.
If you have root access
If you own your server and want to unblock the domain from sending you can edit this file from the TERMINAL in WHM.
follow the instructions:
here you'll find a file for each blocked domain, just delete the file to re-enable SMTP sending for the domain