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[Discuss] PTR records for email

Bill Horne wrote:
> I have a question about PTR records: can anyone confirm or deny that
> some ISP's are refusing mail from any domain without a PTR?

Yes, no PTR record at all will lead to many receivers classifying your
mail as spam.


  Most email mail transfer agents (mail servers) use a forward-confirmed
  reverse DNS (FCrDNS) verification and if there is a valid domain name,
  put it into the "Received:" trace header field.

But it is rare for a legit IP not to have any PTR.

The article also says:

  Some email mail transfer agents will perform FCrDNS verification on
  the domain name given in the SMTP HELO and EHLO commands.

I have only seen one case of this that I am aware of. It wasn't an ISP.
It was Craigslist. I mentioned it in this posting:

My understanding is that best practices for spam filtering say you
should *not* filter on an expectation of the PTR record matching the
EHLO host.

See also:

  There are various reasons why it may be impossible, impractical or not
  desirable to set up forward confirmed reverse DNS in the correct
  domain...It is therefore not recommended to take a failed verification
  as proof of anything. It is however a mandatory internet requirement,
  according to RFCs that each and every host has a valid reverse lookup.

  ...there are systems that take having no reverse DNS, a failed
  verification, reverse DNS in an other domain or something that looks
  like the reverse of a dynamic IP address into account because they see
  a correlation between that and spam, but correlation does not imply

Most of the article deals with lookups performed solely on the client's
IP and information retrieved from DNS. In a bullet list at the end they
repeat the comment about EHLO checks from the previous article, and add
a note about how such checks are contrary to an RFC:

  Some e-mail mail transfer agents will perform FCrDNS verification on
  the domain name given on the SMTP HELO and EHLO commands. This can
  violate RFC 2821 and so e-mail is usually not rejected by default.

If you have an ISP that refuses to give you a custom PTR record, you can
work around that for a price. VPN services, like PRQ:

can get you a static IP and custom PTR starting at $13/month. (Though
you can likely get a mail relay service for less.)


Tom Metro
Venture Logic, Newton, MA, USA
"Enterprise solutions through open source."
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