Once an additional IP range is assigned to your server, you need to create reverse DNS records (aka PTR records) for each IP from this range. This is necessary for IPs not to be blocked by some e-mail internet servers which reject mail from IPs without such records.
1. Go to the Dedicated Servers/Manage Servers menu
2. Choose the server the IP belongs to
3. Click Edit near the necessary IP range in the Extra IP addresses field
4. Click the green arrow opposite to the IP
5. Type in domain name for this IP and click Submit
6. The newly added record will appear on the Edit IP range page.
One more Important thing Setting up reverse DNS records is implemented only for class C network IPs.