This strategy was effective briefly, until the spammers realized that most legitimate mail servers allowed open relaying. Mail servers configured for open relaying would accept any message from anyone, anywhere, and forward it to anyone, anywhere. The spammers quickly took advantage of open relaying. Their only recourse was to block thirdparty.org. Because of this unethical exploitation of open mail relaying, it is now widely recognized by Internet systems administrators that the proper behavior for a mail server is to accept and deliver a message only if it is addressed to or from a local user.
Several websites exist whose sole purpose is to track sites that host open mail relays.
They publish what are essentially blacklists, which more and more system administrators are using to protect their users from spam. Unfortunately, USC is on one of the more popular blacklists because of the spam that has been and continues to be relayed through our hosts. All mail from USC is refused by any site that uses the blacklist. For this reason, all University mail servers should be configured to disallow mail relay. The person in charge of a host allowing mail relay may not even be aware that the machine is configured to allow mail relay.