Well Google unilaterally withdrew their service for ISPs back in 2015 (I think), despite what many assume, it was never a case that VM didn't want to pay them and went cheap.
But have you ever wondered why Google and Microsoft are so much better at spam filtering? Well firstly they have far more technical resources to throw at the situation, secondly they both offer a fully paid-for email service (GSuite and Microsoft 365) and the 'free' versions piggy-back off of the same infrastructure. But definitely in Google's case, for the free gmail service, they effectively read every email which passes through them looking for useful information for advertising purposes - they have a massive, vested interest in being very, very good at working out what emails are.
But it should be trivial to remove spam, after all it's obvious isn't it?
Well not so obvious to a machine which can only go strictly by sets of rules it has been given. I often like it to a situation where your postman, opens each of your letters before you get them, he reads each one, making a careful note of any useful information which can be sold on to marketing companies, and if he or she decides that it's not something you want to receive, simply dumps it in the recycling bin or shredder depending on the settings. Oh and the postman can't use any human intuition, but must religiously follow a set of rules handed down from the Post Office which determines if you receive the letter or not.
