We can block outgoing mails for one particular email account or domain using the cpanel filter rule.
Steps to disable mail for particular Domain.
Cpanel Default filter file. /etc/cpanel_exim_system_filter
1. Copy the cpanel filter file, using default can be replaced on future cpanel update.
cp -rp /etc/cpanel_exim_system_filter /etc/cpanel_exim_system_filter_EDITED
2. Set the copied cpanel filter file on exim configuration file.
Find the line — > system_filter = /etc/cpanel_exim_system_filter
Replace it with — > system_filter = /etc/cpanel_exim_system_filter_EDITED
3. Paste this rule on the bottom of the filter file.
$h_from: contains “@DOMAIN_NAME”
fail text ” Sorry You are not allowed relay emails from the server.”
Replace the word “DOMAIN_NAME” with the exact domain name to which you want to disable outgoing mail.
4. Restart exim to take effect of new settings.
You can use this filter rule to block both incoming and outgoing mails for particular domain.