Open mail relay

An open mail relay is an SMTP server configured in such a way that it allows anyone on the Internet to send e-mail through it, not just mail destined to or originating from known users. This used to be the default configuration in many mail servers; indeed, it was the way the Internet was initially set up, but open mail relays have become unpopular because of their exploitation by spammers and worms. Many relays were closed, or were placed on blacklists by other servers.

History and technology

Until the 1990s, mail servers were commonly intentionally configured as open relays; this configuration was also frequently the installation default setting on UNIX systems. The traditional store-and-forward method of relaying e-mail to its destination required that it was passed from computer to computer (through and beyond the Internet) via modems on telephone lines. For many early networks, such as UUCPNET, FidoNet and BITNET, lists of machines that were open relays were a core part of those networks.Filtering and speed of e-mail delivery were not priorities at that time and in any case the government and educational servers that started the Internet were covered by a federal edict forbidding the transfer of commercial messages.