What is this error?
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 emails are allowed.
How can I fix this?
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 1:45 pm, the block will release at 2:00 pm and the domain will be able to send emails out once more.
If you're not sure exactly what is causing this, you can probably figure it out by using the Track Delivery icon in your hosting control panel (CPanel). When you click the Track Delivery icon, you'll see a field where you can enter a recipient's email address and then click a "Run Report" button to get information about the email sent to that recipient. If you enter nothing for the recipient email address, you'll get back data for all email traffic, and as you look through it you should see groups of bounced messages which can help you determine what sender caused the problem, and why.