[Gllug] E-Mail screening and reverse DNS

[Casper Gasper]

On 10/06/06, Ken Smith <kens at kensnet.org> wrote:
> Hi Guys, I run my own SMTP server (this message will come from it in a
> moment) using Sendmail. DNS is hosted here and at another site. The top
> priority MX record points to my sendmail set up here.
>
> It's been working fine for a few years now with very few outages or
> problems. However, one site I often send e-mail to has just recently
> started rejecting my messages, and messages from there to here are also
> rejected at their end. My messages just vanish into hyperspace without
> response and theirs to here get the following response at their end:
>
> Reporting-MTA: dns; ***smtp.******.net
> Arrival-Date: Mon, 5 Jun 2006 09:42:24 +0100
> Final-Recipient: RFC822; kens at kensnet.org
> Action: delayed
> Status: 4.2.0
> Remote-MTA: DNS; kensnet.org
> Last-Attempt-Date: Mon, 5 Jun 2006 13:52:59 +0100
> Will-Retry-Until: Sat, 10 Jun 2006 09:42:24 +0100
> (I have noticed that something similar is happening when exchanging
> messages with BT Internet recently.)
>
> My guess is that the IT dept at the remote site have set up some
> rigorous reverse DNS validation. The reverse DNS for here will be
> ksmithdsl.claranet.co.uk. So the reverse DNS for my locally hosted
> domain won't check out. My outgoing e-mail goes directly and not via
> Clara's SMTP server.
>

 Is there anything in your sendmail logs?  If not, try telneting to
the mail server in question and manually connecting; you might get a
more helpful error message.

> So my question is: how is this dealt with? What do ISP's do, where, for
> example I imagine that post.demon.co.uk will be the source and sink for
> messages for all kinds of domains that Demon host for their clients. So
> the reverse DNS lookup won't check out. Or am I just being dumb and
> missing something obvious.....

 If you have a fixed IP, ask your ISP; they may change the rDNS to
knetgate.kensnet.org.  If you don't have a fixed ip, that might well
be the problem -- some domains won't send or receive mail from
addresses known to be dynamic.  In general, the best solution for
these is to send the mail via your ISP's servers.

Casper.
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