Pete Barnwell pete.barnwell at
Sun Dec 20 16:14:32 CST 2009

Joe Greco wrote:
>> On Fri, 18 Dec 2009, Jason Bertoch wrote:
>>> Do metrics exist on how many current installs still rely on the implicit
>>> MX?
>> It's very common for email from web servers to be poorly configured such
>> that it uses the webserver's hostname as the return path's mail domain.
> 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:
> Even five years ago, it was considered mildly problematic to deploy a
> hostname where the A pointed someplace incapable of receiving mail,
> since some "products" (you know who you are) were so poorly written
> and still in use that they would connect to the A (or "implicit MX" 
> if you prefer) even in the presence of MX records.
> Now that another five years have passed, it would be interesting to
> see how many antiques are still sending e-mail AND are worth talking
> to.  I'm guessing not many.
> 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?
> ... JG

I'd think this more than common - the  A  record for the domain quite
often is set to point to the same IP as the www. A record where that 
server isn't running an smtp service.
We've certainly got clients who do this, and haven't ever reported it
causing problems = one example :-

banquo>host -t A has address
banquo>host -t A has address
banquo>host -t MX mail is handled by 3
banquo>host -t A has address



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