Is there no way for you to tell the Google software that it is not spam? My email client has adaptive controls, so if I mark a message as not spam it remembers for future use. Also, I can set up more specific controls.
The problem may be the amount of spambots that have previously registered with fictional email addresses or whose email accounts are no longer valid or have full mailboxes. The big clients like Google and Yahoo use the percentage of emails that bounce as an indicator of spam. So if the forum sends out a big batch of emails over to Yahoo and, for a hypothetical example, most of them go to email addresses that don't exist, have been unactive for a long time or have full mailboxes then Yahoo may mark ALL the forum's emails as spam. I have been working on this for some time, by creating a script that automatically processes bounces and deactivates and eventually deletes members whose email addresses bounce. However, depending on how forgiving these automated systems are, they may still hold previous bounces against the forum. In any case, it could be a lot worse. It is possible to get blacklisted for too many bounces, which would mean the email wouldn't even show up in your spam folder! :O Anyway, if you do try to see if there is an easy way to mark any one of the emails as 'not spam' in your client, and if so see if that causes future emails to not be put in your spam box and then see if that helps.
Alternatively, if you know how to check the full source code of the email with headers, you can send that to me (privately! for your own sake) and I can contact the server with it and see if they can give me any more information.
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"That virtue we appreciate is as much ours as another's. We see so much only as we possess." - Henry David Thoreau