I think it's a false economy. At best it is security by obscurity. Google tell you why things are spam and often they even return quite detailed error emails w

I think it's a false economy. At best it is security by obscurity. Google tell you why things are spam and often they even return quite detailed error emails when they don't accept stuff it doesn't benefit an attacker that much. Any decent attacker already knows that they should sign stuff and make identifiers align and can do so trivially easily. People like Yahoo are the opposite and are completely opaque as if they are doing anything that clever. All they can realistically do is check originating IPs, message content, alignment etc. just like everyone else. Since I can still a lot of very decent SPAM in my inbox, their lack of transparency clearly doesn't work so they might as well help legitimate senders to deliver stuff properly.