Stuart D. Gathman wrote:
I keep wishing (as I did years ago) that we could just resurrect
reverse path for MAIL FROM from RFC821. SRS is pretty ugly by
comparison.
Yes. And a bit more liberal, allowing "bounces-to" sent to another
network, unrelated to the sending MTA. At least in theory, not for
all ordinary users of a cheap ISP like me.
If an MTA was to forward mail with an RFC821 style reverse path,
would subsequent MTAs send DSNs back to the right place with
reverse path intact? Or would the reverse path get elided?
It would be ignored, it's a "MUST accept", "SHOULD ignore", and
IIRC "MAY honour" feature in 2821. They'd accept it, but then
they ignore it.
Reviving it is hopeless, the forwarders don't set it, and even if
they'd do it the next hop would ignore the reverse path.
Or from the POV of the next hop: It ignores a reverse path, and
even if it would honour it, the forwarder didn't set it.
They burnt that bridge in RFC 1123 seventeen years ago. Trying
to rebuild it would be a "worldwide upgrade" task, I don't think
there's any chance for it.
Besides the second thing SPF does is to ignore the reverse path
(RFC 4408 2.5 last paragraph). The appeal about this part was
a complaint that the reverse path is not "archaic" in the same
as sense as a %-hack or bang paths, and it was anyway rejected -
the spec. couldn't cram the complete history of e-mail routing
in one paragrah.
Although it's ironic that SPF tries to emulate a feature killed
seventeen years ago. Archaic or not, the reverse path was sound.
Frank
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