What is Reverse DNS and what are the mail server requirements for it?

Reverse DNS is a way of associating an IP address with its hostname. The reverse DNS identifier is contained in the PTR portion of the IP Zone File. The IP Zone File contains all the different ways that your IP and domain name can be associated; each association serves a different need.

• We require that all connecting Mail Transfer Agents have established reverse DNS, regardless of whether it matches the domain.
• Reverse DNS must be in the form of a fully-qualified domain name. Reverse DNS containing in-addr.arpa are not acceptable, as these are merely placeholders for a valid PTR record. Reverse DNS consisting of IP addresses are also not acceptable, as they do not correctly establish the relationship between an IP address and its associated domain.
• Reverse DNS that may be similar to dynamic IP space (containing pool, dhcp, dyn, etc.) may be treated as suspect, and should therefore should be changed to reflect a fully-qualified domain name with standard MTA reverse DNS. [Example: mail.yourdomain.com]
• Reverse DNS must have a proper matching A record for the hostname specified.
  
  

Verifying DNS conformance

Ensure that your PTR and A records are visible by the rest of the world over the Internet, as sometimes they appear fine internally within your organization, but are not propagated over the Internet due to a delegation failure. Use one of the many free web-based tools available over the Internet to verify your reverse DNS records as they are seen by the rest of the world:

• http://remote.12dt.com/
• http://www.dnsgoodies.com/