It can throttle its acceptance of messages from the same IP block There are literally tens of millions of computers infected with malware making them part of a

It can throttle its acceptance of messages from the same IP block There are literally tens of millions of computers infected with malware making them part of a botnet (ex: http://www.csmonitor.com/USA/2011/0629/Biggest-ever-criminal-botnet-links-computers-in-more-than-172-countries http://www.csmonitor.com/USA/2011/0629/Biggest-ever-criminal...). The cost to hire 1 million of these computers (all with unique IP addresses) to send emails out is trivial. You'd be shocked how cheap it is. Now here's the thing... SPAM is pretty easy to spot because someone is trying to sell something. In this case, you're not trying to sell anything. You just need Google to download things. So you send an email with "Vacation Pics" in it. Sure, Google could filter them out but they're all from unique IP addresses from home computers across the country/world and they aren't trying to sell anything. They probably could filter out some of them at the cost of filtering out a lot of false positive legitimate mail. Internet security is complex - you seem to think otherwise.