Fingerprinting the World's Mail Servers (O'Reilly)
This summer, the sales staff at MailChannels came to the dev team with an urgent request: "Can you tell us which companies are running Sendmail? If we could know that, it would be so much easier to sell our Sendmail-compatible product." For those of us who understand the SMTP protocol, the answer was, of course, a resounding "Yes." Most mail servers announce their identity when you connect to them on TCP port 25. The dev team decided that this was a summer science project they just had to get on top of. We even gave the science project a name: PingedIn, and we hope to provide more dynamic content on our skeletal website."
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