[rubberstock]

1) With gmail, they have the foundation for good filters.

2) Google knows the identity of almost any internet user, as shown by the new recaptcha system.

3) They don't allow banned people to recreate new accounts. At least that seems to be a problem for people who were banned from the app store.

If they link this up, they should be able to keep inboxes clean. And if this fails, they just have to offer an option to switch to a whitelist approach for people whose inbox is flooded.