From cPanel version 11.32, a new feature is implemented to limit the ability of compromised sites to send out spam emails. Sometimes customers may trigger this feature due to mass mailing that sends out emails to undeliverable addresses on their list.
As this feature is enabled, you may get the following bounce back.
“Domain has exceeded the max defers and failures per hour (5/5 (26%)) allowed. Message discarded.”
cPanel will regularly monitor the emails sent through all email accounts on your domain, and if, over the past hour, more than 25% of the attempted deliveries have failed, the outbound emails will temporarily be limited.
To solve the issue, remove the following file. /var/cpanel/email_send_limits/max_deferfail_domain.com
(domain.com is generalized. You should be able to find the domain name that happens to face issues here)
and restart the exim service or you may need to disable the option “Rate limit incoming connections with only failed recipients” in Tweak Settings in WHM.
If you still have an issue with this, in Tweak Settings, change the value for “Maximum percentage of failed or deferred messages a domain may send per hour”. The option limits for the maximum rate of a domain’s outgoing mail that can consist of failed or deferred messages. Once the domain exceeds this rate, it is temporarily blocked from sending mail.
If you’re not sure exactly what is causing this, you can probably figure it out by using the Email Trace icon in your hosting control panel. When you click the Email Trace 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 an 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.