Instructions for Email Administrators

Diagnosing  DNS/rDNS issues for Email admins:

Use nslookup to determine whether the IP address for your mail server resolves.
 
--- example ---
c:\ nslookup 174.133.228.34

Server: 192.168.50.35
Address: 192.168.50.35#53
 
Name: ns1.formodaserver.co.uk

--- end example ---
 
If the IP address does not resolve, that is the root cause of the rejection.
 
If the IP address resolves, as in the example above, use nslookup to verify that the hostname assigned to the IP address resolves to the same IP address.
 
--- example ---
c:\ nslookup ns1.formodaserver.co.uk

Server: 192.168.50.35
Address: 192.168.50.35#53
 
Non-authoritative answer:
Name: ns1.formodaserver.co.uk
Address: 78.129.233.101
--- end example ---
 
If the hostname does not resolve, or does not resolve to the same IP address, as in this case, that is the root cause of the problem.  

How to resolve DNS/rDNS mismatch issues:
 

If you manage your own DNS records, you need to update the DNS records for the outbound mail server to ensure that the IP address of the mail server resolves to a hostname, and *THAT* hostname resolves to the same IP address.
 
If you do not manage your own DNS records, you need to contact your Internet service provider and ask the service provider to update the DNS records for the outbound mail server to ensure that the IP address of the mail server resolves to a hostname, and *THAT* hostname resolves to the same IP address.
 
Implementation details vary widely, depending on the OS on which the DNS server is running, the software which is used to provide DNS service, and the tools available to the systems administrator, so we are unable to provide detailed instructions for each environment