ANTISPAM_DNS

The recipient's SMTP blocks your mail because one of these reasons:
- Your HeloDomain in server.tab is no real domain name
- There is no domain name pointing to your IP address
- DNS reverse lookup of the sender's address failed, 
  probably a missing PTR record in your or your ISP's DNS server.

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. 

