If your reverse DNS record does not match the host name of the server, or if the domain given in the reverse DNS record does not resolve back to IP, you will have difficulty sending and receiving mail on your dedicated server. This is why it's critical to have a proper rDNS setup and configuration. Also, you can further white-label your server and separate yourself from the enemies.
Your server has a default reverse DNS record set up which points to the primary domain of your service.
Reverse DNS record update changes take approximately 24-72 hours to propagate.