6.2.7. EXPLICIT PATH SPECIFICATION

At times, a message originator may wish to indicate the transmission path that a message should follow. This is called source routing. The normal addressing scheme, used in an addr-spec, is carefully separated from such information; the <route> portion of a route-addr is provided for such occasions. It specifies the sequence of hosts and/or transmission services that are to be traversed. Both domain-refs and domain-literals may be used.

Note: The use of source routing is discouraged. Unless the sender has special need of path restriction, the choice of transmission route should be left to the mail transport service.