You can limit the number of emails a domain can send per hour. To do so, use the Max hourly emails per domain option under the Mail tab at Home >> Server Configuration >> Tweak Settings. This setting defines a server-wide limit for every domain.

You may further refine this setting by specifying values for an individual package ( Home >> Packages >> Edit a Package) or for an individual account (Home >> Account Functions >> Modify an Account).

You may also specify values for individual domains by editing the cpuser file at /var/cpanel/users/. To do so, add a MAX_EMAIL_PER_HOUR-[$domain] key and specify a value. Remember to replace $domain with the domain you wish to limit. If you make any changes to the cpuser file, make sure to run the /usr/local/cpanel/scripts/updateuserdomainsscript.

After configuring the maximum number of emails a domain on your system can send hourly, configure The percentage of email messages (above the account’s hourly maximum) to queue and retry for delivery setting. You can configure this setting under the Mail tab at Home >> Server Configuration >> Tweak Settings.
When an account exceeds the maximum number of emails, by default, additional messages are queued for delivery and sent the following hour. This setting allows you to limit the number of messages that will be queued by the system.
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Setting the Max hourly emails per domain option to 500 would allow each of the domains you host to send 500 email messages per hour. Assume one of your domains uses a mailing list with 500 members. If this domain sends a message to the mailing list, then sends an additional 25 email messages in the same hour, the domain would exceed the Max hourly emails per domain limit. In this scenario, a domain is sending a high volume of messages; however, these messages are not spam.

For this reason, you can specify a “soft limit” using the The percentage of email messages (above the account’s hourly maximum) to queue and retry for delivery option. If, for example, you have set the The percentage of email messages (above the account’s hourly maximum) to queue and retry for delivery value to 150, the domain can queue up to 250 messages to send in the next hour. In this scenario, the domain is able to queue the additional 25 email messages to send in the next hour.