If you don't have cPanel, follow the information provided in this post.
How to verify Reverse DNS (PTR) is correctly set using SSH:
Enter the following using your forum's IP address:
dig -x 123.123.123.123 +short
You should get back your forum's domain:
host.domain.com.
Enter the following domain for your forum:
dig host.domain.com +short
You should get back your forum's IP address:
123.123.123.123