[jmathai]

TLDR; It's a tough problem to which I have no suggestions.

It's not just Google. Trust me when I say that combating spam is a full time ongoing job that never ever ends and you never ever win.

I do agree that it's the responsibility of the email provider to mitigate spam but it's an incredibly difficult task to accomplish with near zero false positives.

Perhaps Google could do a better job but restricting the "open" nature of signing up for and using email is not the solution. If Google does that then someone else will offer it unrestricted.

It's simply too cheap for spammers to send emails. Identifying usage patterns and rate limiting or gasp charging based on them would be a step in the right direction. But that means losing users which in itself has tremendous cost.