Open mail relay

SMTP server configured in such a way that it allows anyone on the Internet to relay email through it.

Until the 1990s this was the normal configuration for a mail server and was often the default on Unix systems at installation. This was due in part to the fact that in the early days of the internet there were not very many systems connected. Also the few hosts on the internet applied high standards to their conduct, and there wasn't spam in those days.

Once the internet became more of a public service, it wasn't long until it was abused by spammers. However after spammers started abusing open relays to forward their own messages, it became frowned upon by some members of the Internet community to run such a server. The Can Spam Act of 2003 outlaws this use in the United States. Many ISPs use DNSBLs to prevent open mail relay servers to send mail to them.

The most famous mail relay is probably that of John Gilmore, who argues that running an open relay is a free speech issue, and that since he has never sent any spam personally he has a right to configure his computer however he pleases.