Google's spam models freaking suck donkey dick. I use Fastmail but my university uses Google for email. A service we use (Piazza, class Q&A) sends notifications via email. Over the years I have gotten probably about 1k email notifications from them, and replied to a few of them too. Google has recently started marking about 5% of their notifications as spam. 95% of the notifications come through; 5% get rejected. There's no discernible logic to this.
There are organizations and people within the university which have me on their mailing lists; with them too 90% of the emails get through but 5% get randomly marked as spam. And they're perfectly normal emails that Google just decides to move to spam. Even with people that I have had email conversations with, it just spams random stuff.
Every couple of months I check my Fastmail spam folder just for the lolz of seeing what's in there. With Google I have to check spam every day or I'll miss important stuff.
There are tons of people who have posted very similar experiences on HN. On this thread I just saw another person comment that Google's own emails go to spam - I've observed this too on my personal Gmail account which I occasionally check.
The spam filters have a tough job, but other companies are succeeding at keeping all the spam out without a gigantic false positive rate. Google's filter is hilariously bad.