Generic rDNS

This is NOT an issue that directly causes a CBL listing, however, it can lead to a CBL listing via the your mail server's "self-identification" process. This issue is included here for background to provide a wider understanding and help you with possible deliverability issues through other DNSBLs such as SORBS DUL or NJABL DUL, or other anti-spam filtering mechanisms such as the "Enemies List" (which lists generic rDNS patterns).

Many providers will give out IP addresses to end users who expect to run mail servers, and don't bother to set up reverse DNS (rDNS) that accurately reflects the domains the end user will be using on their mail server.

For example, the provider "example.com" sets up this rDNS for a small business that wants to run a mail server:

dhcp-2.3.5.2.foo.bar.example.com

Instead of something like:

mail01.examplecustomer.com

The issue is that the vast majority of mail connections from IP addresses with reverse DNS like that are infected with viruses or spam-sending trojans. In addition, most are on DHCP (dynamically allocated) IP space - which usually have large numbers of infected machines "hopping around" and spewing junk.

"DUL" lists (like SORBS DUL or NJABL DUL) tend to list entire provider ranges where such generic reverse DNS is present.

Many spam filters rely on blocking email with generic reverse DNS too.

It is best that you contact whomever controls the reverse DNS for your mail server (often your provider) and ask them to set the reverse DNS for your mail server to be something sane - eg: "mail01.yourdomain.com"

The CBL does NOT list on this basis, but it can indirectly lead to listings.