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An open relay is an Simple Mail Transfer Protocol (SMTP) email server that allows anyone on the Internet to send messages through it while hiding or obscuring the source of the messages being sent.
Open relays do nothing to identify the original sender of email messages, making them very vulnerable to address spoofing, a technique that alters email headers to appear as though they originated from a source other than the actual source. Although this is how email was initially set up, this type of system is often exploited by spammers.
Open relay is also known as an open relay server, insecure relay, third-party relay, open mail relay and spam relay.
Until the 1990s, open relays were simply SMTP email servers that facilitated email relay between closed email systems. They allowed system administrators to debug email connectivity issues, often by routing email around known problems.
The Internet was originally set up to use open relays. It employed a store-and-forward method of getting email to the intended destination, usually from one computer or server to the next, through the Internet and beyond into local area networks, other networks and eventually to the intended individual users. The computers or servers that functioned as open relays were core parts of the networks. Examples of such early networks were UUCPNET, FidoNet and BITNET.
In the 1990s, unscrupulous senders or spammers rerouted large volumes of email through third-party email servers to avoid detection. Server owners or administrators typically were not even aware of the problem until the system crashed and business was lost. Spammers located vulnerable servers by using automated tools that were readily available for download on the Internet. In effect, these senders hijacked the server, took over the network and computer resources and laundered their spam by making the messages appear to come from legitimate sources. Actually, they were stealing services from unknowing server owners.
Eventually, Internet service providers (ISPs) prohibited their email from going through open relays by using domain-name-system-based blocking lists. Over several years the percentage of mail senders using open relays was reduced from 90 percent to less than one percent. But spammers developed other techniques such as botnets (Internet agents or robots) or zombie computers.