Seems to be quite a lot of people who can't figure out how to turn off the emails when they get new replies on my forum. The forum default is set to notify by email. They also don't understand that you have to disable email notifications on old threads. Guess quite a few users use their spam button to stop them, which in turn can make Gmail classify all forum emails as spam for everyone.
The emails should include a link to disable all new and old thread emails with a confirmation.