However, I disagree with the sentiment that senders can tell
receivers to "kill it, don't pass it on" as expressed here.
This implies that senders control the threshold at which receivers
discard mail. I consider this unrealistic.
I think we all agree that receivers will only do what's in their own
self-interest, and they'll only take senders' advice if it helps that
along.
"It's all spam" is about the simplest useful advice a (non) sender can
give. In my case, which I don't think is unusual, I get buckets of
spam and blowback to subdomains that have never, ever, sent a real
message. The domains are the names of computers on my network, which
were probably scraped out of usenet or mail archive message IDs. If
receivers were to reject or drop all mail purporting to be from those
domains, it would be uniformly better both for the receivers (less spam,
cheap filter) and for me (less blowback.)
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