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Sender IP must resolve
Reverse DNS (rDNS) is a method of resolving an IP address into a domain name, just as the domain name system (DNS) resolves domain names into associated IP addresses. One of the applications of reverse DNS is as a spam filter. Here's how it works: Typically, a spammer uses an invalid IP address, one that doesn't match the domain name. A reverse DNS lookup program inputs IP addresses of incoming messages to a DNS database. If no valid name is found to match the IP address, the server blocks that message.
If you receive any bounce back emails with this error message included:
The Postfix program: host gateway
.activehost.com [69.89.239.177] said: 501 5.7.1 ... Sender IP must resolve
The sender of the original email (which bounced) will need to contact their host and inform them that they need the proper Reverse DNS entries set up for their domain. Many ISP's and Mail hosts require reverse DNS for incoming emails, so users without the proper rDNS will experience problems sending to multiple domains.
Here is a lookup where you can check your own or someone elses reverse DNS entries http://www.dnsstuff.com.
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