Path: utzoo!attcan!uunet!snorkelwacker!tut.cis.ohio-state.edu!ucbvax!CRVAX.SRI.COM!LARSON
From: LARSON@CRVAX.SRI.COM (Alan Larson)
Newsgroups: comp.protocols.tcp-ip
Subject: re: MX Records
Message-ID: <639253683.40000.LARSON@CRVAX.SRI.COM>
Date: 4 Apr 90 18:28:03 GMT
Sender: daemon@ucbvax.BERKELEY.EDU
Organization: The Internet
Lines: 43

Recently I wrote:
>> Suprisingly, it is a technical error for the MXer of highest preference
>> to not be the host itself.  This would result in an empty MX list after
>> removing irrelevant RRs in the MXer of highest preference.  This is an
>> error condition, although it is noted that "974 points out that "extremely
>> persistent mail systems might want to try a delivery to REMOTE's address..."

Michael Stein caught me for not stating one of my assumptions when he asked:

>What about the case where there are "non-internet" paths to
>the host from the MXer of highest preference?

Michael is quite correct in noting this.  RFC974 expects that the
MXer of highest preference (which it calls LOCAL) will know how to
deliver the mail without resorting to the domain name system.
My assumption was that the last hop to the final destintion host was
also a DNS resolved internet hop, which is clearly not always the case.

I have attached the relevant paragraph from RFC974 at the end of this
message for those who do not have a copy.

	Alan



   After removing irrelevant RRs, the list can again be empty.  This is
   now an error condition and can occur in several ways.  The simplest
   case is that the WKS queries have discovered that none of the hosts
   listed supports the mail service desired.  The message is thus deemed
   undeliverable, though extremely persistent mail systems might want to
   try a delivery to REMOTE's address (if it exists) before returning
   the message. Another, more dangerous, possibility is that the domain
   system believes that LOCAL is handling message for REMOTE, but the
   mailer on LOCAL is not set up to handle mail for REMOTE.  For
   example, if the domain system lists LOCAL as the only MX for REMOTE,
   LOCAL will delete all the entries in the list.  But LOCAL is
   presumably querying the domain system because it didn't know what to
   do with a message addressed to REMOTE. Clearly something is wrong.
   How a mailer chooses to handle these situations is to some extent
   implementation dependent, and is thus left to the implementor's
   discretion.
	
-------