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I was just reading a book,HeadFirst Networking,and came across the following line "The mail will also be rejected if the sender's reverse DNS lookup IP address does not match its real IP address".I don't get the exact meaning of the line.The mail server will do a reverse DNS lookup by querying the DNS server for the domain name by giving it the IP address.How does this,in general help to block a person from sending fake mails.

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How does this,in general help to block a person from sending fake mails?

Basic principle: Reverse DNS lookups increase confidence that the person can be held accountable for their actions. Confidence is increased if the forward DNS entry matches the reverse entry; the reason for additional confidence is that the person sending from that IP address has enough control over this infrastructure to make the forward and reverse DNS name match. Forward records are easy to generate using services like DynDNS, but reverse DNS records are harder to fabricate.

  • Many mail servers are configured to check whether the forward DNS name matches the reverse DNS name before putting the DNS name in the mail header field. Some mail servers will even reject this mail on the principle that the person isn't really in control of the network infrastructure they are sending mail from.
  • In a similar way, when the client IP connects to a mail server and issues the HELO or EHLO command to send mail through the server, some mail servers validate that the address provided in those commands likewise matches the revere DNS name.
  • Many mail-servers are configured to reject mail if the reverse DNS record isn't mapped or is just a generic name like (nyc-dialup-192-45.domain.local)

See the wikipedia Anti-Spam DNS Techniques article for a detailed description of these principles.

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Not all mail servers do this, but it is becoming more and more common among the big ESP providers, the best write up I seen explaining this from Bruce Barnes. See the second post located here: rDNS Discussion

He goes in depth about what else is being required of email senders at the major ESP. It's going to become super critical that you have everything set up correctly when sending mail.

He references testing your rDNS and other authentication methods using the Unlock The Inbox Email Authentication Test Tool

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