Na página 44 do white paper "Anti Spam - Melhores Práticas" da IceWarp (PDF):
To make certain that someone cannot breach the system and send messages to server accounts via the server domain name, administrators should select the option, “Reject if originators domain is local and not authorized.” This option prohibits spammers from spoofing legitimate accounts such as PostMaster and Admin, and will eliminate uncertainty in the end user.