Offline routing uses a modified version of Postal Services over Swarm. Swarms are logical groupings of Service Nodes, based both on their public keys and the hash of the block that their staking transaction first appeared in. Each swarm has a swarmID and consists of nine nodes. The sending user must calculate which swarm the receiver belongs do, done using the public key. They can then anonymously route a message through the network to a random Service Node in that swarm. That Service Node will then distribute that message to the other 8 nodes in the swarm, all of which store the messages for their allocated time-to-live. The receiver can then query any two nodes in the swarm for messages that he can decrypt.