Network Working Group                                    Craig Partridge
Request for Comments: 974                 CSNET CIC BBN Laboratories Inc
                                                            January 1986

                   MAIL ROUTING AND THE DOMAIN SYSTEM


Status of this Memo

   This RFC presents a description of how mail systems on the Internet
   are expected to route messages based on information from the domain
   system described in RFCs 882, 883 and 973.  Distribution of this memo
   is unlimited.

Introduction

   The purpose of this memo is to explain how mailers are to decide how
   to route a message addressed to a given Internet domain name.  This
   involves a discussion of how mailers interpret MX RRs, which are used
   for message routing.  Note that this memo makes no statement about
   how mailers are to deal with MB and MG RRs, which are used for
   interpreting mailbox names.

   Under RFC-882 and RFC-883 certain assumptions about mail addresses
   have been changed.  Up to now, one could usually assume that if a
   message was addressed to a mailbox, for example, at LOKI.BBN.COM,
   that one could just open an SMTP connection to LOKI.BBN.COM and pass
   the message along.  This system broke down in certain situations,
   such as for certain UUCP and CSNET hosts which were not directly
   attached to the Internet, but these hosts could be handled as special
   cases in configuration files (for example, most mailers were set up
   to automatically forward mail addressed to a CSNET host to
   CSNET-RELAY.ARPA).

   Under domains, one cannot simply open a connection to LOKI.BBN.COM,
   but must instead ask the domain system where messages to LOKI.BBN.COM
   are to be delivered. And the domain system may direct a mailer to
   deliver messages to an entirely different host, such as SH.CS.NET.
   Or, in a more complicated case, the mailer may learn that it has a
   choice of routes to LOKI.BBN.COM.  This memo is essentially a set of
   guidelines on how mailers should behave in this more complex world.

   Readers are expected to be familiar with RFCs 882, 883, and the
   updates to them (e.g., RFC-973).

What the Domain Servers Know

(以下，略)