ANTISPAM_DNS

The recipient's SMTP blocks your mail because
- there is no domain name pointing to your IP address or
- DNS reverse lookup of the sender's address failed.

In most times this is the result of a non-existing or malformed 
sending address. If the address is OK, you should check your DNS 
config. 

You'll find some useful DNS diagnostic tools at
http://www.dnsstuff.com/

Some mail servers try to do a bidirectional name resolution: 
from IP to HOSTNAME, and from HOSTNAME to IP. 

So if you don't have 'reverse DNS lookup zone', the mail server can't 
resolve your HOSTNAME and assumes your address as invalid.

Reverse lookup zone entries can be delegated by your ISP. 

