I find the "anything I don't like goes into spam" is among the biggest problems for classifying spam using rules shared across users, and has been forever. We support training the classifier in our products, via a couple of different mechanisms, but don't really make it too evident how to enable it and use it, because the result is often a lot of confusion as users break their classifier over time by putting legitimate (but no longer wanted) mail into the spam folder.

Unsurprisingly, GMail also exhibits this problem. I have to check my spam folder regularly because it gets a ton of false positives these days. Nearly always newsletters and such I've signed up for. Another category of common false positive, which is admittedly harder to cope with, is moderation notifications from my WordPress sites...it's often actually a spam post, but it seems to learn the shape of those messages and start to treat them all as spam, even the legitimate ones.

Spam filtering has always been, and continues to be, hard. It seemed, for a brief shining moment a few years ago, that spam was on the run...but, it's come roaring back in recent years, I guess as cloud-based mail services made it feasible for spammers to bounce from one provider to another, to keep sending through "known trustworthy" senders. There probably ought to be some way to punish those providers that look the other way when used for spam, but blocking them completely is likely to impact non-spammers (but might be a necessary pain to reduce spam).