For cPanel & WHM version 68
(WHM >> Home >> Security Center >> SMTP Restrictions)
This interface allows you to configure your server so that only the mail transport agent (MTA), Mailman mailing list software, and the
root user can connect to remote SMTP servers.
This will deny other users and services the ability to bypass your mail server to directly send mail, which is common practice for spammers.
Manage SMTP restrictions
Click Enable to deny users the ability to bypass your mail server to send mail. To allow users the ability to bypass your mail server, click Disable.