Hi, when we register on the forum we are automatically subscribed to every topic we post to.
What happens is that there are people who are annoyed with this and for whatever reasons don't come change their subscription settings and instead report the forum notifications as spam. So in gmail it says that many people reported those messages as spam.
Maybe the default setting for new users should be to not be automatically subscribed to the topics they post to (in case there is this option; I didn't check but I suppose there is).
P.S. - I checked the options and maybe new users should still be subscribed to the topics they post to but without email notifications.