In your broadband control panel, click Edit Reverse DNS.
Enter a fully qualified domain name against the relevant IP address. It is important to fully qualify the domain name by adding a full stop to the end of the domain name.
For example, if you have a mailserver configured on a static IP of 184.108.40.206 , you would create an A record in your domains zone file for this such as mail.myserver.com IN A 220.127.116.11
The output from the UNIX host command would be
~$ host mail.myserver.com mail.myserver.com has address 18.104.22.168 mail.myserver.com mail is handled by 10 mail.myserver.com
However, if you did host 22.214.171.124 the command would return
~$ host 126.96.36.199 188.8.131.52.in-addr.arpa domain name pointer 184.108.40.206.dsl.gradwelldsl.co.uk.
Setting the FQDN in the Broadband Control Panel to mail.myserver.com. would return
~$ host 220.127.116.11
18.104.22.168.in-addr.arpa domain name pointer mail.myserver.com.
This would ensure that mail delivery is not rejected by servers due to the reverse DNS not matching.