Have you been able to resolve your email issues?
You need to assure that your forum generated emails have the following correct:
In addition to these, your Return-Path header needs to be accurate and come from an actual email address. On some hosting platforms, the Return-Path can get mangled and cause you issues.
Also you did not mention if you are on shared or dedicated hosting? If on shared hosting, your server's IP reputation is also shared. A low IP reputation can cause delivery issues.
You can check your IP reputation at
https://www.senderscore.org/
If you have issues at Gmail, Yahoo and Hotmail. They all offer various postmaster services to help you get feedback about your email. I've dealt with cases where a legitimate forum user was marking new post notices as spam instead of opting out.
If you have a high volume of forum emails, you may need a dedicated email channel for those emails. You can do this by either using your server, a third party service or setting up a cheap VPS to relay the email.
Lastly, if you have domain/IP reputation issues, fixing it can take time. I usually see improvements in 5-15 days post email optimization but have had cases take 8 weeks to resolve.