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Open mail relay
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<p>An open mail relay is a <a href="/facts/Simple_Mail_Transfer_Protocol/TeCRKDpJ">Simple Mail Transfer Protocol</a> (SMTP) <a href="/facts/Server_(computing)/dEaYFD7r">server</a> configured in such a way that it allows anyone on the <a href="/facts/Internet/Xw1rVvJU">Internet</a> to send <a href="/facts/Email/6pD9GLtI">e-mail</a> 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 <a href="/facts/Spamming/Qc9IL60W">spammers</a> and <a href="/facts/Computer_worm/9qKR1Hx5">worms</a>. Many relays were closed, or were placed on <a href="/facts/Blacklist_(computing)/ShhQWdCd">blacklists</a> by other servers. </p>