[kid64]

Here's one little-known tip I learned from Fastmail support some time ago: it's only possible to train the spam filter using their Web UI. If you're using a 3rd-party client, simply moving messages to your spam folder will have no effect on your filter quality. So to truly flag a message as spam, you must switch to your browser, login to FM, and handle it from there.

[amelius]

But I don't want to mark everything that should not grab my immediate attention as spam.

Some stuff like promotions or social messages deserve their own folder. Google did this right.

[theshrike79]

Google does it right because they don't respect your privacy.

They have millions of users and they use aggregated data from all of them to manage the automatic categories.