Disable/suspend email service for one domain
From version 56, cPanel provides API support to disable email service for domain or email accounts. Please note this limit cannot be override by the user from their cPanel. This can be very helpful if there is any spamming activity and you just want to disable the email service feature for the domain for the time being.
You can run the below commands through SSH console
To suspend email service for a the cPanel user “aacenyor”
whmapi1 suspend_outgoing_email user=aacenyor
For unsuspending the account, please run the below command:
whmapi1 unsuspend_outgoing_email user=aacenyor
If you are not comfortable with SSH, you can also call these through browser XML.
Example:
To suspend
https://hostname.example.com:2087/cpsess##########/xml-api/suspend_outgoing_email?api.version=1&user=aacenyor
where replace hostname.example.com with your hostname
replace cpsess########## with WHM session id.
To un suspend
https://hostname.example.com:2087/cpsess##########/xml-api/unsuspend_outgoing_email?api.version=1&user=aacenyor
For older versions cPanel, you may refer to this below forum from cPanel. There is a workaround
https://forums.cpanel.net/threads/how-do-i-block-one-domain-from-sending-email-from-my-server.223731/#post920912