Gmail has probably the best spam filtering in the world, but you seem to think it's flawless, and it's not. I just now signed into my old Gmail account for the first time in ages. There's spam in there, and recent too. Roughly one per day by the looks of it -- messages that aren't newsletters, or from previous contacts, or anything that I signed up for.

And spammers aren't even that smart, they're just numerous and financially motivated.

If somebody wanted to annoy someone else with this, they could.

And, again, even if this doesn't work against Google directly, it certainly would work against other service providers who decide to follow Google's lead.

Heck, just look at the recent popularity of the WhatsApp spam that spread malware to tons of people (including Cryptolocker in some cases), or the "Secure Document" phish that made the rounds in Gmail in September.

Ok I see your point; but I want to add that one of the reasons your old account has spam is because you don't use it; I don't know what algorithm they are using but it certainly relies on usability data; I don't know exactly but could be: what language you usually speak in; what hours of the day you receive and read important email, what subjects you read about, etc etc.

Yeah, that could be. I have to spend a significant part of my time now dealing with spam (sysadmin), and Gmail's handling of spam is years ahead of most of the tools I have available.