Ditto for transits across .forwards, aliases, mailing lists
and courtesy
copies. There are ways around this, for instance by encoding consent
tokens into Message-IDs and then looking for them in In-Reply-To and
References: headers, but this of course breaks for those MUAs
that don't
generate In-Reply-To: or References: headers...
It is the lack of references and reply to headers that is the
problem. message ids are created at the server if the client did
not generate one.
Even outlook can probably be made to generate the right stuff.
The problem with outlook are mostly caused by the old X.400 core
getting in the way of the SMTP support.
2) Because of forged sender addresses, bounces for spam are
often not
deliverable, so the spam ends up as double bounces in the
postmaster's
mailbox.
Or in the case of a hijacked address they end up hitting an
innocent party.
Phill
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