Another great one is when you're having a conversation with someone, and after several back and forth emails, the next one goes to spam. Sent from the same device, same headers, etc. Presumably it triggered some keywords. But I mean, come on! Obviously if I've replied to this person four times in a row, their fifth email is not spam.

I realize that everything takes time to implement, and developer time is not infinite, but this one seems like pretty low-hanging fruit.