What is rDNS?
Reverse DNS is an IP address to domain name mapping - the opposite of forward (normal) DNS which maps domain names to IP addresses.
For example, Forward DNS for "abc.com" pointing to IP address "22.214.171.124", does not necessarily mean that reverse DNS for IP "126.96.36.199" also points to "abc.com". This comes from two separate sets of data.
We are doing some additional checks for Flockmail servers to prevent incoming email spam on the basis of configuration of rDNS.
Error message that would be displayed for a misconfigured rDNS:
[Default] 450-'4.7.25 Client host rejected: cannot find your hostname, [xxx.xxx.xx.xx]'
What is the reason for a bounce back?
This bounce-back message is generated when the emails get rejected when the rDNS record of the IP of your email server does not match the forward DNS or if there are no rDNS record setup at all.
How do you fix this?
If you need a PTR Record(rDNS) set up for your mail server, you'll need to contact your mail service provider as this is something they would set up for you.
If your mail server's domain name were mail.yourdomain.com, then you would ask your internet service provider(ISP) to set up a reverse PTR record for your mail server and give them the IP address of the server.