Isn't that what spam filters do though? Or is there some technicality since they're technically not silently dropped but instead in a folder that you don't care about.

Yeah I think it's something like that. Actively rejecting on an SMTP level is fine too, since the sender is now aware. Just dropping it completely silently seemed to be the problem from what I recall.

I'll have to do some further reading on this. Do you vaguely remember where you read this and why it was not allowed?

I feel like it was either here on HN, or on some article linked here, a few weeks/months ago. But I could be wrong... vague recollection at the moment. I don't think the reason was mentioned explicitly but to me it's pretty obvious that it would be a question of accountability.