The email filtering system, like those of most commercial ISPs, requires sending MTAs (Mail Transport Agents) to be correctly configured with reverse DNS. Reverse DNS is a way of associating an IP address with its domain name. Any legitimate MTA that sends Internet email should have a PTR associated with it. Generally speaking, this PTR record would be set by the ISP responsible for maintaining the mailserver. If you do not understand or are not capable of configuring a PTR, you should not attempt to configure your own email server.

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.

The filtering system does 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 domain and IP address.

Reverse DNS that may be similar to dynamic IP space (containing pool, dhcp, dyn, etc.) may be treated as suspect. Therefore should be changed to reflect a fully-qualified domain name with standard MTA reverse DNS. (It is also noted that email from dynamic IP space is unlikely to be accepted by filtering systems.)