[Mimedefang] DNS and MX records

Jan Pieter Cornet johnpc at xs4all.nl
Wed May 10 16:58:34 EDT 2006


On Wed, May 10, 2006 at 02:48:42PM -0400, Kevin A. McGrail wrote:
> Further, I believe there really is a "standard' to publish a blank MX record
> at priority 0 but I think I heard about it from Jan-Pieter Cornet.  Anyone
> know if this has a real RFC or anything?

It was described in a now-expired ietf document draft-delany-nullmx-00.txt,
still available at:
http://ietfreport.isoc.org/all-ids/draft-delany-nullmx-00.txt

It works even for clients that do not recognise the protocol, because
"." has no A/AAAA records associated with it, so it cannot be used to
deliver email to it.

However, (and I just found that out by googling for the exact reference),
it is a bad idea because of largely uncachable, unnecessary lookups to
the root nameservers, as explained in email threads here:
http://www.mhonarc.org/archive/html/ietf-mailsig/2005-04/msg00015.html
and here:
http://www.ops.ietf.org/lists/namedroppers/namedroppers.2005/msg00939.html

As far as I know, there is no newer standard to mark a domain as
not used for email (maybe SPF -all).

But since we implemented it anyway, I do log such rejections, where the
_sending_ domain is marked as not-for-email using MX 0 .

We rejected almost 0.09% of the emails using this method yesterday
(6695 out of 7452398 mails). 5951 rejects were for angelfire.com, 75 for
altavista.se, and then there are 43 other domains with fewer rejections.
Of those 43, 13 domains had "yahoo" in the name, totalling 152 rejected
emails (Mark Delany, who initially proposed this, works at yahoo).

-- 
Jan-Pieter Cornet <johnpc at xs4all.nl>
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