Does your forum use some of new domains, not old ones like .com, .org or by countries? Most mail filters consider something like .co (pure one), .click, .talk etc as possible spam — and too bad, but filters are mostly right.
I don’t use Outlook, but can’t you whitelist senders there? But if you are wondering other users there is not too much to do if a forum has DKIM etc right. False spam alerts have been as old issue as spams that aren’t stopped.
Google seems to be quite generous and is letting thru almost anything nowadays.