The Free On-line Dictionary of Computing (30 December 2018):

source route
mail path
source routing

    An electronic mail address which specifies the
   route the message should take as a sequence of hostnames.
   It is called a source route because the route is determined at
   the source of the message rather than at each stage as is now
   more common.  The most common kind of source route is a UUCP
   style bang path, "foo!bar!baz!fred'.  The RFC 822 syntax,
   "@foo:@bar:fred@baz", is seldom seen because most systems
   which understand RFC 822 also perform automatic routing based
   on the destination hostname.  A third, intermediate, form is
   sometimes seen: "fred%baz%bar@foo.com".