When your system decide to reject message as it consider it as SPAM what
would have to do remote system? Bah... exactly the same as that what you
refusing to do: it will generate bounce.
Not so. Quite often, the program knocking on your door is malware. This
will not generate a bounce. By not accept-then-bounce a message, this
will already cut down on backscatter.
I fill that I was misinterpreted. It is normal that if you do not wont to
message you just reject it. But I cannot agree with any system that
TEMPFAIL message, it has doubted about, just in case to be able reject it
later, when other host will retry to send it again.
Also important: when backscatter is reported (for instance spamcop) let
the other host deal with those reports, not you.
It is discussable, but I would prefer RFC section specifying way to check
authentity of mail in place of point of view of private company saying
Not accepting a message _is_ an important decision. You are saying you
do NOT accept responsibility for that message, you don't trust it.
Very well then reject it! My point is that it is very bad idea to make
work OTHER host because of YOUR doubt.