I had the same problem many times in the past, with different websites using different scripts (not only forum scripts). Unless if PHP function mail() is disabled by your hosting provider, here are the most common reasons that your email are not deliver to your members:
- Your domain is a new registration and thus still does not have a reputation. Mail servers like Gmail, Outlook, Yahoo, most probably will mark them as spam.
- Your DNS records are not completed. eg SPF, DMARC Policy etc are missing
- Your Email (or IP) is already in Blacklist.
You can do some tests at:
mxtoolbox.com
https://tools.dnsstuff.com/
As underline notice, dealing with emails, is a hell.