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).