Nolisting is the name given to a technique to defend electronic mail domain names against e-mail spam.
Contents
Each domain name on the internet has a series of one or more MX records specifying mail servers responsible for accepting email messages on behalf of that domain, each with a preference. Nolisting is simply the adding of an MX record pointing to a non-existent server as the "primary" (i.e. that with the lowest preference value) - which means that an initial mail contact will always fail. Many spam sources don't retry on failure, so the spammer will move on to the next victim - while legitimate email servers should retry the next higher numbered MX, and normal email will be delivered with only a small delay.
Implementation
A simple example of MX records that demonstrate the technique:
MX 10 dummy.example.com. MX 20 real-primary-mail-server.example.com.This defeats spam programs that only connect to the highest priority (lowest numbered) MX and do not follow the standard error-handling of retrying the next priority MX.
Drawbacks
Similar techniques
There are alternate techniques that suggest "sandwiching" the valid MX records between non-responsive ones. Some variants also suggest configuring the highest-numbered hosts to always return 4xx errors (i.e. "retry later").
Like "nolisting", the greylisting technique relies on the fact that spammers often use custom software which will not persevere to deliver a message in the correct RFC-compliant way. It is however, a more sophisticated technique that needs specialist software to track which senders and IPs that it has encountered - hence "nolisting"s title of "The Poor Man's Greylisting".