How would you solve the spam problem at Gmail?

You start with SPF, DKIM, DMARC and IP reputation.

But then I'd use the moat GMail enjoys as a primary mail destination by statistically looking at the frequency an incoming email's metadata and content similarity has recently arrived at GMail as a whole, then moving suspicious arrivals into a progressive exposure pathway that tentatively delivers a small percent to known currently-active users (eg mobile GMail app open on screen and unlocked) and see what percentage are flagged as spam, archived or deleted. Then use that real time feedback to vary the delivery flow of remaining messages to inbox vs spam.

First and foremost you want to make sure SPF, DKIM, and DMARC are valid from the sender's mail server

https://www.smartertools.com/blog/2019/04/09-understanding-s...

Then, email filtering companies use things like IP Address of the sender, keywords, and NLP, and other tools to determine if the email has spam and assign it some kind of score to it.

You can use the + feature to generate different email addresses and then chuck anything coming to the root address. That way when you get spam on a particular address you just block that one address.

Apart from a preapproved list of contacts, I would ask for a fee to deliver each message and only high bidders get to pass. Which is more or less what they re doing

Perfect task for AI. Especially considering that Google has more resources for it than bad actor.

This sounds like a product-manager-interview question.

Interview question? ;)