I believe this is functionally equivalent to the "block 25 and consider
SMTP dead" FUSSP.
It's worth noting that each "newer" system is being systematically attacked
as well. It isn't really a solution, it's just changing problem platforms.
The abuse remains.
Yes, but the ownership of the problem is better defined for messages -inside-
If you've got tens of millions of users on your IM service, you can start
using statistical techniques on your data to identify likely spam/ham,
and (very importantly) you are able to cut individual users off if they're
doing something nasty. Users can't "fake" their identity like they can
with email. There's no requirement for "broadcasting" messages a la email
lists (which btw is touted as one of those "things that break" when various
anti-spam verify-sender proposals come up.)
Besides - google has a large enough cross section of users' email to do
these tricks. I'd love to be a fly on the wall at google for just this