This is an outgoing spam prevention measure introduced since cPanel v32. It limits the maximum amount of failed or deferred emails. When more than 90% of the emails sent from your domain fail or defer, our mail server temporary blocks the domain from sending further messages. You would then get a bounce-back message that contains an error similar to this one:
Domain yourdomain.tld has exceeded the max defers and failures per hour (10/10 (90%)) allowed. Message discarded.
Simply put, this error appears after you send multiple email messages to email addresses/recipients that are unavailable, don't exist or reject your emails.
For example, if you send 10 emails and 9 of them bounce back, either because the email recipient is invalid or unreachable, your domain would reach the limit of 90% of failed/deferred messages.
What causes it?
This limitation can occur when you send an email to multiple recipients (including CC and BCC) and most of them are invalid or unreachable. In this case it's recommended to send emails separately, one for every recipient.
The limitation can also 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 frequent cause for hitting this limit, as these are sent back to all your incoming emails, including spam emails that frequently use fake/inexistent email addresses.
What is the purpose of this limitation?
The main reason behind this limitation is to prevent spam from getting sent through our network. Most spam campaigns have a high bounce-back rate due to many 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).
When does the limitation expire?
The only way to get your domain unblocked is to wait for up to 1 hour and not send any further emails. 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.
How can I check which email recipients have failed or deferred my emails?
You can check which emails have failed or deferred in cPanel under Email -> Track Delivery.
How can I prevent this?
To avoid getting your domain blocked again, you should verify that all email recipients are valid and that you can deliver your emails to them. If you have a large mailing list, you should check periodically if the email addresses in your list are correct and still exist. The most effective way of doing this is to ask your subscribers/recipients to (re-)confirm their email addresses.
If you have email auto-responders or email forwarders, you should try to reduce them or - if possible - disable them completely.
If sending emails to multiple recipients (including CC and BCC), you should try to send the email separately to each recipient.
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. We can scan your account for malware on request. If your hosting account or email account are compromised, 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 your knowledge and permission. Once you make sure that your hosting account, computer and network are clean, you should change the password of your hosting account and email account(s), as a precaution.
Should you still reach this limit after you check all of the above possible causes, please contact our technical support department.
