🚀 go-pugleaf

I believe Tom Fitzgerald is referring to RFC 1123 (section 5.2.16),
which reads as follows in DISCUSSION:

              Although source routes are discouraged within the Internet
              (see Section 5.2.6), there are non-Internet mail
              environments whose delivery mechanisms do depend upon
              source routes.  Source routes for extra-Internet
              environments can generally be buried in the "local-part"
              of the address (see Section 5.2.16) while mail traverses
              the Internet.  When the mail reaches the appropriate
              Internet mail gateway, the gateway will interpret the
              local-part and build the necessary address or route for
              the target mail environment.

              For example, an Internet host might send mail to:
              "a!b!c!user@gateway-domain".  The complex local part
              "a!b!c!user" would be uninterpreted within the Internet
              domain, but could be parsed and understood by the
              specified mail gateway.
              An embedded source route is sometimes encoded in the
              "local-part" using "%" as a right-binding routing
              operator.  For example, in:

                 user%domain%relay3%relay2@relay1

              the "%" convention implies that the mail is to be routed
              from "relay1" through "relay2", "relay3", and finally to
              "user" at "domain".  This is commonly known as the "%-
              hack".  It is suggested that "%" have lower precedence
              than any other routing operator (e.g., "!") hidden in the
>>>>>>>>>>>   local-part; for example, "a!b%c" would be interpreted as
>>>>>>>>>>>   "(a!b)%c".
		
              Only the target host (in this case, "relay1") is permitted
              to analyze the local-part "user%domain%relay3%relay2".


If you look at the above example, it would appear to be in error,
because they do in fact give % higher precedence than !, even though
they claim that one should not.  However, the example is consistant
with their usage of "%" as a right-binding operator.  The easiest way
to get around this mess is to keep the following two issues in mind:

[1]	The "%-hack" is unofficial at best, and not recommended.  If
	you can find a better way, use the other address.

[2]	When they mentioned %, the authors suggested that it receive
	lowest precedence.

It is important to restate the group's major wish that % or any other
form of source routing NOT be used at all for internet mail
addressing (Section 5.2.6):

         DISCUSSION:
              The intent is to discourage all source routing and to
              abolish explicit source routing for mail delivery within
              the Internet environment.  Source-routing is unnecessary;
              the simple target address "user@domain" should always
              suffice.  This is the result of an explicit architectural
              decision to use universal naming rather than source
              routing for mail.  Thus, SMTP provides end-to-end
              connectivity, and the DNS provides globally-unique,
              location-independent names.  MX records handle the major
              case where source routing might otherwise be needed.

My point in mentioning the above paragraph is that any site that is
only reachable via the %-hack should be converted into an MX site.
-- 
Eliot Lear
[lear@turbo.bio.net]