Ned,
I was one of the people consulted on the source routing issue in the Host
Requirements RFC. I've been with it since the standards were created. I was
one of the people who lobbied for the Host Requirements RFC wording on source
routing as it now stands. Please don't try to second-guess what our
motivations and intentions were.
Source routing ala the RFC-821/2 at-domain-list syntax does not work.
Let me repeat:
Source routing ala the RFC-821/2 at-domain-list syntax does NOT work.
The at-domain-list syntax is OBSOLETE and ARCHAIC. The SOLE use of this
syntax in today's world is that of an SMTP client pre-pending its own name to
a return-path of an external-origin message for the purposes of tracing. An
implementation MUST be able to parse the at-domain-list syntax, but it is free
to ignore any information purportedly conveyed therein.
This is the true state of affairs today, and it exists because the DCA, then
the governing authority of the Internet, decreed that a name to the right of
the "@" in an e-mail addres MUST be valid within the Internet naming authority
(this being the DNS). This had the effect of making the at-domain-list syntax
USELESS for ALL practical purposes.
I created the so-called "%-hack" precisely because DCA had abolished the
previous multiple-@ syntax by administrative decree (because it caused BBN's
Hermes mail UA to crash and BBN wouldn't fix it) in RFC-733 days. I was a
user of the multiple-@ syntax and had an emergency need for a replacement.
When the at-domain-list syntax was created I considered using it, but again a
DCA decree prevented it. I could not have "@SU-SCORE:M(_dot_)MRC(_at_)SU-LOTS"
because
SU-LOTS was not within the Internet naming registry. Therefore, I remained
with "M(_dot_)MRC%SU-LOTS(_at_)SU-SCORE" which worked and was valid within the
standards.
The primary advantage of the "%-hack" is that it is a HOST-SPECIFIC function
and thus is neither sanctioned nor forbidden by the standards. As a formerly
HEAVY user of the "%-hack", I can state that the need for it has largely
evaporated with the widespread adoption of MX records. The %-hack is needed
only when:
1) political considerations prevent a host's registration in the DNS
2) a user has a specific reason to route through a host, e.g. to work around
broken mail software
In BOTH cases, it is desirable that this be a PRIVATE function and NOT,
repeat, NOT something mandated by the standards.
There is no reason why "%" can not be used as a character in a mailbox name.
-- Mark --