Disable localhost relay Mail

Attention: All VPS or Dedicated Server WHM cPanel  administrators/owners.  
You can disable the local relay message by applying the following settings:

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. 

/usr/local/cpanel/bin/tailwatchd --disable=Cpanel::TailWatch::Antirelayd