[ On Mon, July 6, 1998 at 20:39:53 (-0700), Bill Studenmund wrote: ]
> Subject: Re: Greg Woods - please fix your mailer! 
>
> Why? Why do I have to have two entries (MX and A) for a host, if that host
> is perfectly capable of receiving its own mail?

Because a "host" is far more than a mailer alone, and a mailer is but
one service a host might offer, but may not offer.  MXs point at hosts,
mail is delivered via MXs.

> I like the idea of verifying the DNS entry for a host, but why reject the
> case when you get no MX and an A? You are in a position to send back an
> error message (well, as good a position as you are with an MX record). ??

A tremendously large number of A RRs in the DNS point at dial-up hosts
which do not (and cannot, by definition, if they are dynamically
assigned) run SMTP servers.  If an SMTP client were to use such an A RR
in their SMTP sender envelope address to any receiving SMTP other than
their own locally authorized outgoing SMTP relay gateway, then it will
be impossible to return any mail to them.  At least with a valid MX
there's a far better chance that the assumption a connection to the SMTP
port at the destination MX host will succeed.

In the bigger picture there's also the fact that if your sender address
is not pointing at a domain with a valid MX then there's a very good
chance that your Reply-To: header (or lacking one the From: header)
doesn't point at a domain with a valid MX either.  Anyone not using an
e-mail address with a valid MX address is either bogus, or living in the
wrong decade.

-- 
							Greg A. Woods

+1 416 443-1734      VE3TCP      <gwoods@acm.org>      <robohack!woods>
Planix, Inc. <woods@planix.com>; Secrets of the Weird <woods@weird.com>