RFC 821 indicates that the
<forward-path> argument of the form
"@ONE,@TWO:JOE@THREE" to indicate intermediate routing hosts (
@ONE,@TWO) and the final receiver of the email (
JOE@THREE). See 3.6, Relaying: http://tools.ietf.org/html/rfc821
RFC 2821 dictates that if an SMTP server is sent a forward-path of
@ONE,@TWO:JOE@THREE that the email could be directly routed to
JOE@THREE - bypassing the intermediate hosts - by performing a DNS MX lookup on
THREE. See 3.7, Relaying: http://tools.ietf.org/html/rfc2821
I believe I understand RFC 2821 correctly: the DNS MX lookup would simply return the IP of the SMTP server for the domain (e.g.
@THREE here). Is this right? What I don't understand at all, however, is how email would be routed under RFC 821. This specification was written up before DNS existed, so I presume any host names were specified in a hosts.txt file? How were emails with forward paths routed?