There's no "tricking" the forum software, it's going to perform the same way unless/until there's an upgrade or host change. What changes is how your ISP, mail client, etc., view email coming from this forum.
Looking at the DNS records, here's my guess on what happened recently. The forum was moved to a new IP last Saturday (Dec 4th):
https://viewdns.info/iphistory/?domain=tropicalfruitforum.comThat new IP is shared by 150 other websites:
https://viewdns.info/reverseip/?host=192.252.156.17&t=1All those sites (most likely) share the same mail server and definitely are subject to the same content classification/filtering many ISPs and 3rd party sites do these days. Sites sharing the same mail server can be sending out spam and/or unwanted mass emails and the mail server IP goes on black/block lists, everybody suffers. If some site on the IP has mature content or other filtered content, all sites can also be dinged. ISPs (Comcast has particularly sucked in the past) can have internal blocklists that you don't even know your site is on. Getting off those lists is a constant battle.
The solution is to be on your own IP and using your own mailserver. That solves these problems and comes with additional benefits, but it's also a few magnitudes more expensive. Not cost-prohibitive in today's environment, but more members would need to contribute during the yearly fund drive.