Updated expired links, minor formatting. Originally Issued Third-party relay occurs when a mail server allows an external mail client to forward mail for processing and delivery without any checks to ensure that the client is a legitimate user. An external mail client is one that is outside of Penn.
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. ISC may disconnect such machines from PennNet until the problem can be corrected in order to ensure reliable Internet connectivity. Many commercial businesses and Internet Service Providers have taken steps to harden their systems against spammers so spammers appear to now be targeting university e-mail servers. If you run a mail server that accepts third party relay, spammers and intruders will eventually find your system and use it for sending spam or for harassment.