An open relay (sometimes called an insecure relay or a third-party relay) is an SMTP e-mail server that allows third-party relay of e-mail messages. By processing mail that is neither for nor from a local user, an open relay makes it possible for an unscrupulous sender to route large volumes of spam. In effect, the owner of the server -- who is typically unaware of the problem -- donates network and computer resources to the sender's purpose. In addition to the financial costs incurred when a spammer hijacks a server, an organization may also suffer system crashes, equipment damage, and loss of business.
In the past, open relays were used intentionally, for example, to facilitate mail relay between the separate closed e-mail systems (such as UUCP or FidoNet) or to allow network administrators to debug mail connectivity issues and route mail around known problems. However, the Internet has expanded enormously since then, and the potential for abuse has expanded accordingly. Spammers have learned that by hijacking open relays, they can in effect "launder" their spam, delivering their unsolicited e-mail while spoofing their identity to make their messages appear to come from a legitimate source.
Spammers are able to locate accessible third-party mail relay servers by using automated tools that are readily available on the Internet. By relaying mail through several open relay mail servers at the same time, it is possible to flood the Internet with large amounts of junk mail in a very short period of time before being detected. Spammers who use third-party mail relays not only damage the reputation of those whose servers they have hijacked, clog networks with junk mail, and frequently crash servers -- they are guilty of breaking the law because technically, they are stealing services.
The Mail Abuse Prevention System (MAPS) is one of several organizations that seeks to control the problem of open relays, though educating the public about the danger, and through publishing a blacklist of organizations whose mail hosts allow third-party relays. The relay feature is a part of all SMTP-based servers, which means that most modern e-mail servers, if unprotected, are vulnerable. According to MAPS, because spammers use automated tools to search the Internet for vulnerable servers, an open relay will eventually be found and used. To avoid allowing spammers free access to their resources -- and to help stem the Internet's flow of spam -- MAPS urges administrators to turn off the relay option on their servers.