[ronaldx]

So stuff which appears to be spam is bounced, with a return message.

On reflection, this does seem like a better system. I had imagined a silent fail as per other popular e-mail hosts.

[snori74]

Yes, but the "bounce message" comes to the sender from their own mail system, saying "could not deliver".

This is important, because the receiving system doesn't have a guaranteed way of reaching the sender - modern spam typically fakes the "From:" and <return-path> fields - so well-run systems nowadays never try to do so for fear of generating what's referred to as email backscatter spam.