421 DNS:NR
 http://postmaster.info.aol.com/errors/421dnsnr.html


EXPLANATION:

Reverse DNS lookup for your IP address is failing. 

SOLUTION:

Reverse DNS is a way of associating an IP address with its domain name. The reverse DNS identifier is contained in the PTR portion of the IP Zone File. You may look at your reverse DNS using our reverse DNS search tool. 

If you are on a dynamic IP address, please call your ISP and request a static IP address with proper rDNS before attempting to send mail to AOL through that server. 

If you are on a static IP address, please call your ISP or DNS provider and ask them to establish a reverse DNS record for your IP address, reflecting your correct domain name.

Failure! Unfortunately we were unable to resolve Reverse DNS for the IP address you entered. Contact your ISP or e-mail administrator to modify these settings. Also please note the following points: 



AOL 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 DNSes containing in-addr.arpa are not acceptable, as these are merely placeholders for a valid PTR record. Reverse DNSes consisting only of IP addresses are also not acceptable, as they do not correctly establish the relationship between domain and IP address.

$> nslookup 212.6.225.4
Note:  nslookup is deprecated and may be removed from future releases.
Consider using the `dig' or `host' programs instead.  Run nslookup with
the `-sil[ent]' option to prevent this message from appearing.
Server:         217.172.166.7
Address:        217.172.166.7#53

** server can't find 4.225.6.212.in-addr.arpa: NXDOMAIN

$>