Kelson wrote:
>
> Who said anything about spam from an authorized source?


I was misled by SPF... sorry.

> The problem *being discussed* is spam with a forged sender address,
> causing bounce notices to go to an innocent third party.
>


which is caused by "accept then bounce" implementations, something that
is considered "bad" by many people since some time now. fixing these
implementations is far simpler than requiring every domain on earth to
decide that its mail should go through few relays.