Re: Last Call: draft-klensin-rfc2821bis

David Morris wrote:
> 
> On Fri, 28 Mar 2008, Keith Moore wrote:
> 
>>> If there were some serious technical consequence for lack of the MX record I would be
>>> all for specifying its use. Operational practice with A records shows that
>>> there is no real issue,
>> only if you ignore the problems that have been observed and the
>> increased likelihood of such problems occurring in IPv6.
> 
> Perhaps you could help us out and share a reference to documentation of
> such problems. I for one have not personally observed any problems related
> to using the A record to locate a mail server when there is no MX.

part of the problem is that most hosts don't wish to receive mail but 
there is no way to indicate this to a mailer that is trying to deliver 
mail to them.  if the host has an A record, under the current 
specifications, a mailer that gets a message (presumably spam) with a 
recipient address at that domain is expected to try to deliver that 
message.  and unless that host implements a dummy SMTP server that 
refuses all incoming mail with a permanent error the sending mailer will 
keep getting "connection refused" - which is treated as a temporary 
error and the sending mailer will keep retrying.  this clogs up mail queues.

and the dummy SMTP server works, but it consumes resources on the host 
and eats bandwidth on the network.  having a way to say "don't send this 
host any mail" in DNS seems like a useful thing.  and we simply don't 
need the fallback to AAAA because we don't have the backward 
compatibility issue that we had when MX records were introduced.

Keith
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