RE: Last Call: draft-klensin-rfc2821bis

Keith Moore wrote:
> Tony Hain wrote:
> > Your arguments make no sense. Any service that has an MX creates
> > absolutely no cost, and the fallback to AAAA only makes one last
> > attempt to deliver the mail before giving up. Trying to force the
> > recipient MTA to publish an MX to avoid delivery failure on the
> > sending MTA is useless, and in no way belongs in a standard document.
> > MX records are an operational optimization, nothing more.
> 
> that's completely incorrect.
> 
> what MX records mean is that a domain name used on the right hand side
> of an email address need have nothing at all to do with any host or
> other service that has the same domain name.  in particular the servers
> and resources associated with that email address don't have to share
> the
> same host, network, addresses, user community, administration, or
> anything else.  (except that the administration of the DNS zone and RRs
> associated with that domain is the same for both)
> 
> in short, MX records decouple mail domain names from host names.  and
> this turns out to be a very useful thing to do.  e.g.
> 
> 1. a domain used for mail that doesn't correspond to any actual host
> 
> 2. a host that doesn't want to source or receive mail
> 
> 3. when it is desirable to associate email and other services with the
> same domain name, and yet not have all of those services hosted on the
> same cpu or at the same address.
> 
> for instance, the email for network-heretics.com and the web server for
> the same domain are hosted by entirely different companies on different
> networks -- because I couldn't find a hosting company that did an
> adequate job of both at a reasonable price.  and yet it's very useful
> to
> have them both associated with the same domain name.

That is all well and good, but it is completely of value to the receiving
MTA, and under their complete control. There is nothing that requires a
receiving MTA to follow this model, despite what others may see as value.
Defining the facility is what the standards need to do. Dictating
operational practice without cause is what a standard needs to avoid doing. 

> 
> > The function of mail delivery is between IPv4/IPv6 endpoints, and how
> > those endpoints find each other is orthogonal to the actual service
> > of mail delivery. Having the document state a prioritization between
> > 2 of the possible methods is pushing the edge already
> 
> that's an incorrect way to characterize what is going on, because an A
> record is only a valid destination for mail to a particular domain if
> no
> MX records exist.  if even a single MX record exists, it's incorrect to
> route the mail based on an A record, even if an attempt to relay the
> mail via the listed MX resulted in a temporary error.
> 

I agree that if an MX exists, the operator of the receiving MTA has stated
its expectations, and the sending MTA needs to oblige. That is not the same
as mandating that every organization has to follow the same model. 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, and that anything that does come up is under the
control of the impacted party with a clear mechanism to resolve it. 

Again, the text is fine as it is.

Tony

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