This allows SMTP server to be configured in such a way that it allows anyone on the Internet to send e-mail through it. This can blacklist the server’s IP address
To disable Open Relay on the cpanel server you can follow following simple steps:
Login to WHM.
Set this setting to Bounce / Fail.This should turn off the open relay on the server.
“Bounce” / “Fail” is usually the best choice if you are getting mail attacks.