<messaging> 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
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: "firstname.lastname@example.org".