Enable SMTP Authentication on cpanel
POP before SMTP authentication allows you to send messages via a POP mail server for 30 minutes before you will need to re-authenticate a session through SMTP.
POP before SMTP authentication is disabled in WHM by default. To enable this option, navigate to WHM’s Service Manager (Home >> Service Configuration >> Service Manager) and select the Antirelayd checkbox.
While POP before SMTP authentication is secure but if the customer will be able to check the email from an office, the current public IP belongs to that office IP has been added to email white list and any one from that network can be send email using this smtp server without any authentication for 30 minutes.
To disable the feature “Authenticate POP before SMTP” in WHM/Cpanel server by simply executing the following command.
To Enabling SMTP authentication