One of the main policies currently used by email servers in SPAM combat is the rDNS check.
Using only the rDNS check, up to 70% of SPAM cases can be eliminated.
For this reason, many providers reject e-mail from servers without the correct rDNS record.
Below is a very simple example of how rDNS works:
Reverse DNS is a DNS record called PTR and is intended to return the host / domain name associated with an IP. It is useful for verifying the “authenticity” of IP.
See in the example below, the query made on the site ipok.com.br for the mx server of Terra ISP:
Now this example of IP without rDNS:
How to set up a rDNS record
To create a rDNS record, you must have control of the authoritative nameserver server.
In the vast majority of cases, only ISPs have this access. Therefore you should open a call in the technical support of your provider, requesting the creation of a PTR record for the IP address X.X.X.X resolving for yourdomain.example.com.
SPFBL antispam system policy for IPs without rDNS:
The SPAM combat in SPFBL system does not block email messages sent by IPs without the back side configured. The SPAM SPAM intelligent identification algorithms use other variables that, together, will determine if the message is being sent by a spammer or not.
However we strongly recommend the correct configuration of the reverse DNS, since the lack of it greatly increases the likelihood that the sending MTA will be listed in the blacklist.