Section Appendix C. Source Routes

Historically, the <reverse-path> was a reverse source routing list of
   hosts and a source mailbox.  The first host in the <reverse-path> was
   historically the host sending the MAIL command; today, source routes
   SHOULD NOT appear in the reverse-path.  Similarly, the <forward-path>
   may be a source routing lists of hosts and a destination mailbox.
   However, in general, the <forward-path> SHOULD contain only a mailbox
   and domain name, relying on the domain name system to supply routing
   information if required.  The use of source routes is deprecated (see
   Appendix F.2); while servers MUST be prepared to receive and handle
   them as discussed in Section 3.3 and Appendix F.2, clients SHOULD NOT
   transmit them and this section is included in the current
   specification only to provide context.  It has been modified somewhat
   from the material in RFC 821 to prevent server actions that might
   confuse clients or subsequent servers that do not expect a full
   source route implementation.
   For relay purposes, the forward-path may be a source route of the
   form "@ONE,@TWO:JOE@THREE", where ONE, TWO, and THREE MUST be fully-
   qualified domain names.  This form is used to emphasize the
   distinction between an address and a route.  The mailbox (here, JOE@
   THREE) is an absolute address, and the route is information about how
   to get there.  The two concepts should not be confused.
   If source routes are used, RFC 821 and the text below should be
   consulted for the mechanisms for constructing and updating the
   forward-path.  A server that is reached by means of a source route
   (e.g., its domain name appears first in the list in the forward-path)
   MUST remove its domain name from any forward-paths in which that
   domain name appears before forwarding the message and MAY remove all
   other source routing information.  The reverse-path SHOULD NOT be
   updated by servers conforming to this specification.
   Notice that the forward-path and reverse-path appear in the SMTP
   commands and replies, but not necessarily in the message.  That is,
   there is no need for these paths and especially this syntax to appear
   in the "To:" , "From:", "CC:", etc. fields of the message header
   section.  Conversely, SMTP servers MUST NOT derive final message
   routing information from message header fields.
   When the list of hosts is present despite the recommendations above,
   it is a "reverse" source route and indicates that the mail was
   relayed through each host on the list (the first host in the list was
   the most recent relay).  This list is used as a source route to
   return non-delivery notices to the sender.  If, contrary to the
   recommendations here, a relay host adds itself to the beginning of
   the list, it MUST use its name as known in the transport environment
   to which it is relaying the mail rather than that of the transport
   environment from which the mail came (if they are different).  Note
   that a situation could easily arise in which some relay hosts add
   their names to the reverse source route and others do not, generating
   discontinuities in the routing list.  This is another reason why
   servers needing to return a message SHOULD ignore the source route
   entirely and simply use the domain as specified in the Mailbox.