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 until the next hour.
This happens when a domain or cPanel account sends out a number of emails that either failed or deferred within the last hour.
If the number of failures or defers exceeds the server limit, no further emails will be processed.
The blocked domains can send emails again at the top of every hour. For example, if you received this error at 1:15 pm, the block will be released at 2:00 pm, and the domain will be able to send emails once more unless the previous messages keep deferring; you'll trigger that error again (even if you didn't send new messages during this hour) until all deferred messages are delivered or the server stop retrying.
What is a deferred message?
A deferred message is the message that the recipient's server didn’t accept from the first time and rather returned a temporary failure. This technique is used by some email servers to mitigate spam messages since most spammers won’t retry to send a deferred message.
