source route
An {electronic mail address} which specifies the
route the message should take as a sequence of {hostname}s.
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".