Attention: All WHM cPanel administrators/owners of VPS or Dedicated Servers.
The following settings can be used to disable the local relay message:

To begin, ensure that AntiRelay is turned on (remove the tick) >> Go to WHM >> Service Manager >> AntiRelay >> (check to make sure it is not ticked)

Second, go to WHM >> Main >> Tweak Settings >> Initial default/catch-all forwarder destination. *Set this setting to Bounce / Fail

Then, make SMTP authentication mandatory for all mailbox or email users on the server.
Go to SSH and run the command below.

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