Re: Last Call: draft-klensin-rfc2821bis

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.

> 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.

Keith
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