block emails with no valid reverse DNS

[Matt Kettler]

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Vasantha Narayanan wrote:
> Hi,
>
> I want to block emails from servers which do not have a valid reverse DNS
> lookup.  I would like to be able to do this without using a DNSBL server,
> but merely using dns.   Can you tell me:
> 1. How this can be done using Sendmail?

Word of warning. Strangely, there's a reasonable number of small companies, and
some not so small that get hit by this. There are fewer and fewer now that AOL
blocks such things, but be aware of it.

I've found that techonline.com, pparadise.com, and legitimate newsletter
remailers savvis and navisite all suffer from this. Most of what seems to get
caught by it is legit product newsletters, which are no major loss, but be aware
of them.

Here, I wind up greylisting them using milter-greylist ACLs. You're welcome to
use this trick:

# note: requires extendedregex option
# supported by milter-greylist 2.0b3 or higher.
# greylist unresolvable hosts
acl greylist domain /\[[0-9]{1,3}\.[0-9]{1,3}\.[0-9]{1,3}\.[0-9]{1,3}\]/

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