Source : Free On-Line Dictionary of Computing

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%[email protected]".