When you use a host-based mail client like Elm, your mail host is your SMTP server by default. When you use a desktop mail client like Netscape Messenger or Eudora, you have to configure your mail client to let it know which server will be the SMTP server and actually deliver your mail message for you. Third-party relay occurs when a mail server allows an external mail client to forward mail not destined for delivery to any direct users of that server. There are sites that will test whether your mail host allows third-party relaying or not.
Intruders use it to try to embarrass a site by spamming with an embarrassing or offensive message.
Intruders also use it to harass some other site by sending thousands of e-mail messages to that other site. When spammers discover campus mail servers with third-party relay, the Penn host often becomes clogged with spam, resulting in lost or late mail deliveries and cleanup work required to remove the spammed mail from the systems without destroying legitimate mail. As a stopgap measure, we have restricted SMTP on ISC Networking mail servers so that it will only relay messages that are being sent from within the upenn.edu domain, including the modem pool, ResNet and Greeknet. This will block all third-party relaying including legitimate relaying by people who have accounts on ISC Networking mail servers but are using a mail host outside of Penn. Configuring your mail client If you have an account on ISC Networking mail servers but are using a mail client through an ISP or host not within the upenn.edu domain, you should check your mail client's configuration and make sure that you are using your remote mail host as the SMTP server and not an ISC Networking mail server.