Disable localhost relay Mail
Firstly, Make sure the AntiRelay is Enabled (remove tick) >> Go to WHM >> Service Manager >> AntiRelay >> (make sure its NOT ticked)
Secondly, Navigate to WHM >> Main >> Tweak Settings >> Initial default/catch-all forwarder destination.
//Set this setting to Bounce / Fail.
Then, Force all mailbox or email users in the server to use SMTP Authentication.
Go to SSH and run following Command.