What’s the difference between a public vs private blocklist?

Public blocklists are openly available for anyone to query. Spamhaus, SpamCop, and Barracuda publish their data and provide lookup tools. You can check your status and understand why you are listed.

Private blocklists are internal lists maintained by individual organizations. Gmail and Microsoft operate extensive internal reputation systems that effectively function as private blocklists. You cannot query these directly.

Private lists present challenges because you cannot confirm your status or understand listing criteria. You infer problems from delivery metrics and bounce messages rather than direct lookup.

Some providers share limited information through Postmaster Tools or SNDS, but this visibility is restricted compared to public blocklists.

Public registries post their lists for all to see. Private ones operate behind closed doors, revealing problems only through their effects.