[Gllug] E-Mail screening and reverse DNS

[Ken Smith]

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.

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.....

Ken
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