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.