Clients who setup your own email server using Microsoft Exchange or Domino mail server, should check if your Reverse DNS is properly setup. Find out what is the fixed IP address of your mail server. Enter the IP address in http://postmaster.aol.com/tools/rdns.html.
DNS Check
If the result returned no valid hostname of your mail server, please inform the service provider of your IP address to request this setup to be done. Without this, outgoing email from your mail server can be marked as Spam as your IP address is not resolving to a valid hostname.
“When you register a domain name with a domain registrar, you usually become responsible for that Forward DNS domain. In DNS terms, the domain is delegated to you. However, you are not also responsible for your reverse records. Your Reverse DNS records are still most likely to be the responsibility of your hosting facility or ISP. To make changed to your Reverse DNS PTR records, you must contact the company where you get your IP addresses from, usually a hosting facility or an ISP.”
Extracted from http://www.tech-faq.com/reverse-dns.shtml