Patent ID: 7660993

Claim:
A coordinating cancellation server of a digital delivery system, configured for executing the steps of: communicatively coupling a first of a plurality of cancellation servers connected through the coordinating cancellation server to at least one database comprising a plurality of unique identifiers for cryptographic puzzles; receiving at a first cancellation server an identifier associated with a cryptographic puzzle, the puzzle being attached to a digital object received from a sender, the digital object being an electronic mail message intended for delivery from a sender to a recipient distinct from the sender; validating the received identifier by verifying that the identifier does not exist in the at least one database associated with the first cancellation server connected to said coordinating cancellation server; querying the coordinating cancellation server from the first cancellation server for the address of a second cancellation server having a database of unique identifiers; receiving the address of a second cancellation server from the coordinating cancellation server; validating the received identifier further at the first cancellation server by directly querying the second cancellation server and verifying that the identifier does not exist in the database associated with the second cancellation server; and upon validating, canceling the cryptographic puzzle by storing in the at least one database an entry comprising the identifier or information derived from the identifier, and transmitting to the recipient an ACCEPT response if the identifier is validated.