For instance, what would happen if mail servers provided feedback to
both senders (on a per message basis in the form of NDNs)
Well, since 95% of all mail is spam, and all the spam has fake return
addresses, you'd increase the amount of bogus NDNs by more than an
order of magnitude. No thanks.
Incidentally, on a bad day I already get 400,000 NDNs from mail that I
didn't send, just from the minority of MTAs that send NDNs in response
to spam now. This is not a hypothetical problem.
R's,
John
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