Gateways from RFC 821 to other protocols
(essentially, a UNIX delivery agent is such a gateway) tend to throw
away envelope (To) addresses and keep header addresses only.
the envelope addresses of other recipients are typically unavailable
to such gateways anyway. SMTP doesn't communicate the envelope address
of each recipient to each MTA - the only envelope addresses sent to
an MTA are those for which *that MTA* is responsible for delivery.
That's precisely the reason that the MAIL FROM address is exposed in the
message header as return-path - so the address can be used for automatic
responses that are generated after the message leaves the transport/delivery
system.
Ah, this is new in RFC 2821. Nice, however it doesn't seem to be
implemented widely.
no, it's in RFC 821 also. however it's true that many MTAs don't
implement it correctly.
Keth