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Eleventh Hour Security+ by Ido Dubrawsky

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SMTP open relays

Organized crime is a significant driving force behind SPAM (unsolicited electronic mail), with research from Cisco showing that SPAM accounted for 90% of e-mail messages sent over the Internet during 2008. Most SPAM is sent from open SMTP relays existing on corporate networks or home computers without the knowledge or consent of the owner. An SMTP open relay is a mail server that accepts e-mail from anyone on the Internet through it. Mail servers should restrict e-mail destined to or originating from known users only, but an open relay allows anyone to send mail through it.

When an attacker relays a mail message, they bounce it off an open mail relay that in turn forwards the message to an address of the attackers choosing. ...

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