Where is the evidence that this is true? It doesn't seem that likely to me that so few people are not using gmail-style spam filtering that spammers couldn't f

Where is the evidence that this is true? It doesn't seem that likely to me that so few people are not using gmail-style spam filtering that spammers couldn't figure out how to target those of us who aren't (the filtering that I could have used but turned off is more random than helpful and I would guess this is the case on many non-Google mail providers). Even with Google running a huge portion of all email there are still a huge number of people with email not provided by Google. An alternative narrative is that previously email servers were supposed to accept almost anything from everyone and spammers took advantage of this. From this perspective it is the relatively recent requirement of sender verification that made the key difference and allows Spamhaus to work (obviously they need to do something to determine who to block). This perspective seems to match what I am seeing.