Simply put, this error appears after you send multiple email messages to email addresses that are unavailable or don't exist.
This is a security feature introduced since cPanel 11.32. It limits the maximum amount of failed or deferred emails. When more than 40% of the emails sent from your domain fail or defer, our mail server temporary blocks the domain from sending further messages. You'll then get a bounce back message that contains an error similar to this one:
Domain domain.com has exceeded the max defers and failures per hour (4/10 (40%)) allowed.
For example, if you send 10 emails and at least 4 of them bounce back because the email recipient is invalid or unavailable, your domain would reach the limit of 40% of failed/deferred messages.
This error can occur when you have one or more email forwarders directed to an external email account (e.g. from your domain to a Gmail address). If the remote mail server rejects your forwarded emails, every email you receive to the forwarded email address would count as a failure/deferral if the remote mail server rejects it. In this case, you would have to reduce your number of email forwarders, or contact the remote email service provider.
Email auto-responders are also a main cause for reaching this limit, as these are sent back to all emails you receive, including spam emails that mostly use fake/inexistent email addresses.
The main reason behind this limitation is to prevent spam from getting sent through our network. Most spam campaigns have a high bounce rate due to the high rate of fake or invalid email recipients. By limiting the rate of failed or deferred emails, we can quickly and effectively limit the damage caused by spammers and maintain a good reputation for our IP addresses by keeping them off the blacklists (Real-time Blackhole Lists such as SpamHaus or SpamCop).
The only way to get your domain unblocked is to wait up to 1 hour and don't send any further emails during this time. The mail server has an hourly monitoring system, so if you've reached the limit at 10:15, the domain will be automatically unblocked at 11:00. Please refrain from sending emails while your domain is limited, otherwise, the counter will reset for another hour.
To avoid getting your domain blocked again, you should verify that all email recipients are correct and that you can deliver your emails to them. If there are just a few email addresses, you can check them in cPanel by using the Email Trace tool. If you have a large mailing list, you should check periodically if the email addresses in your database are still correct and still exist. The most effective way of doing this is to ask your subscribers/recipients to confirm their email addresses (again).
If you have email auto-responders or email forwarders, you should try to reduce them or - if possible - disable them completely.
Another important step is to check your account for old or vulnerable scripts and update them immediately. You should then check if there are any suspicious or malicious files under your hosting account or on your computer that could be the source of a spam campaign. You can use the virus scanner from cPanel to scan your files. If your hosting or email account are hijacked, this could be the reason why your domain often reaches the limit of failed or deferred emails.
You should also scan your own computer or network for viruses, malware or spyware in order to make sure that there's nothing sending emails using your email account without permission.
Finally, after you make sure that your hosting account, computer and network are clean, you should change the password of your email account(s), just as a precaution.
Should you still reach this limit after you check all above causes, please contact our technical support department.