How to define a Reverse DNS Blacklist that can be used by MailEnable to block spammer addresses


This article outlines how to integrate your own Reverse DNS Black List (RBL) with MailEnable.


MailEnable allows for two types of RBL lookups; these are called Zone lookups and Host lookups. As such, MailEnable performs two very different types of lookups depend on whether the DNS blacklist provider entry is defined as a Zone lookup or a Host lookup.

Host Lookup
A Host lookup is when a DNS server (as defined as the Name Server value in the MailEnable Administration Program) is used to check for the existence of the relevant IP address where the mail is coming from. The Sender's IP Address is converted into reverse byte order in following format: (where IP address is p1.p2.p3.p4) If a PTR record is returned from the DNS server for the address, then the address will be blocked.

Zone Lookup
A Zone lookup relies on using the DNS hosts as defined for the SMTP Connector properties. These servers will in turn be queried for PTR records within the namespace defined as the "Zone" value in the MailEnable Administration Program.

Assuming the Zone name is "", then the following PTR lookup would occur: (where IP address is p1.p2.p3.p4)


To define a blacklist, ensure that the SMTP connector is configured to use the specified DNS Server (which can in turn refer any lookups to upstream DNS - this is desirable anyway because of caching benefits)*. Then create a namespace/zone corresponding to the zone name as defined in the "Zone" value in the MailEnable Administration Program. Then add PTR records under that zone for the IP addresses that are to be banned.

Note: *There is no current way to have reverse DNS blacklisting use different DNS servers (for zone lookups) to those used by the SMTP connector. In any case, it is preferable to have a local DNS server configured either on the mail server or on the same local network to facilitate caching.

Configuring DNS and DNS Zones is dependant on a sound understanding of DNS principles and specific to the DNS product that is being used. For more information on configuring DNS, refer to the vendor provided documentation. 


Please also see the following article in relation to configuring Reverse DNS Blacklists:

Product:MailEnable (Custom: Custom: Custom: Custom: Custom: Custom: Custom: All Versions)
Class:HOWTO: Product Instructions
Revised:Wednesday, May 4, 2016