Introduction
Some users want their accounts to utilize POP authentication before usage of SMTP. Below is how to enable this functionality. Please note that this can lead to mail abuse due to when someone authenticates, the IP of the user is whitelisted for future email for the next 30 minutes and this allows someone on the same network to send without restriction.
Procedure
To adjust this, you would need to login to WHM, and from Home > Server Configuration > Tweak Settings, enable the following.
"Allow users to relay mail if they use an IP address through which someone has validated an IMAP or POP3 login within the last hour (Pop-before-SMTP)"