Unlisting
Unlisting was a proposed experimental method of defending electronic mail users against e-mail spam. The suggestion was to carefully track the order in which a host's MX records are connected to. This technique would be a more complex cousin of Nolisting, a method in which fake MX records are used to deceive connecting hosts. Nolisting is based on strict adherence to the standards outlining mail server actions as outlined by RFC 2821 (Simple Mail Transfer Protocol) which specifies that a client MUST try and retry each MX address in order.
A flaw in the Unlisting method makes it "not recommended," according to Unlisting.org:
Unlisting was developed under the assumption that the SMTP client connection must originate from the same IP address when retrying. A close reading of the protocol does not support this conclusion, however.
See also
- Greylisting
- Nolisting
- Port knocking