You can try the following steps to block smtp of a single user:
Login Server from root SSH and run below commands:
1. #touch /etc/blockeddomains
2. #echo “domain.com” >> /etc/blockeddomains
Please replace domain.com with your domain name.
Then login WHM and follow below step:
3. Go to “Exim Advanced Editor > Section: CONFIG scroll down to “Add additional configuration setting”.
Add the line at end:
domainlist blocked_domains = lsearch;/etc/blockeddomains
Then scroll down to the Section: ROUTERSTART which is below the democheck entry and add:
driver = redirect
# RBL Blacklist incoming hosts
domains = blocked_domains
data = :fail: Connection rejected: SPAM source $domain is manually blacklisted.
Conclusion: this blog help you for block mail Send of a single user in WHM.
If you have any type of queries regarding to block mail Send of a single user in WHM so please write down the your valuable comment on comment box.