Re: sink.arpa question

[Tony Finch <dot () dotat at>]

On Sun, 20 Dec 2009, Joe Greco wrote:
> It is very difficult to measure how many current installs rely on the
> implicit MX, as someone else noted.
>
> On a somewhat different angle of attack: [...]
>
> That suggests that it might well be fine to point A at something that
> is not capable of receiving SMTP, as long as you have MX records.  An
> arrangement that should always have been practical, of course.
>
> Is anyone actually doing this?

I can't quite answer your question yet, but here are some related numbers.

I analysed the mail domains used in envelope return paths of 10 days of
traffic from the start of this month (before the end of term - I work for
Cambridge University). This totalled 2473192 messages from 98825 domains
after spam filtering. The data is rather skewed: 43295 domains were used
in only one message each.

The breakdown of these domains from the DNS point of view is:

total: 98825
broken: 897 - neither A nor MX records
no A: 18687
no MX: 6282
mismatch: 56244 - A and MX point to different hosts
partial: 380 - A is not a subset of MX
match: 16335 - A is a subset of MX

Note that there is some difference between the validation done by the DNS
software I used for this analysis and that done by our MTA, and over a
week has passed since we accepted these messages, which is why the
"broken" count is non-zero.

I did this using about 150 lines of perl and `adnshost -f -a`; I don't
have a handy concurrent SMTP tool so the full analysis will take more
work...

Tony.
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