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. -- f.anthony.n.finch <dot@dotat.at> http://dotat.at/ GERMAN BIGHT HUMBER: SOUTHWEST 5 TO 7. MODERATE OR ROUGH. SQUALLY SHOWERS. MODERATE OR GOOD.