How do I setup SMTP Restrictions?
You can setup SMTP Restrictions by logging into WHM and heading to Security Center > SMTP Restrictions.
Inside the interface click the Enable button to start the service.
SMTP Restrictions is a feature in WHM that stops users from bypassing the mail server to send mail. This service is useful because bypassing mail servers is a common practice of spammers. By setting up restrictions you’ll only receive mail from root connections, Mailman mailing list software and mail transport agent (MTA) to remote SMTP servers.