Mailboxes

Send and receive interchain messages
The messaging API is powered by Mailbox smart contracts, which expose an on-chain API for sending and receiving interchain messages. A Mailbox is deployed on every chain Hyperlane supports.
The network of Mailboxes facilitates the connective tissue between blockchains that developers leverage to create interchain applications, and add interchain functionality to their existing applications. If you'd like to learn more about them, read on!

Dispatch

To send interchain messages, developers call Mailbox.dispatch().
This function takes as parameters the message contents, the destination chain ID, and the recipient address. Each message get inserted as a leaf into an incremental merkle tree. Storing messages in a merkle tree allows Hyperlane's proof-of-stake protocol to trustlessly verify fraud proofs.
/**
* @notice Dispatch the message it to the destination domain & recipient
* @param _destinationDomain Domain of destination chain
* @param _recipientAddress Address of recipient on destination chain as bytes32
* @param _messageBody Raw bytes content of message
* @return A unique message ID.
*/
function dispatch(
uint32 _destinationDomain,
bytes32 _recipientAddress,
bytes calldata _messageBody
) external returns (bytes32);

Process

Relayers deliver messages to their recipients by calling Mailbox.process().
This function takes as parameters the message to deliver as well as arbitrary metadata that can be specified by the relayer. The Mailbox will pass the message and metadata to the recipient's Interchain Security Module for validation.
If accepted, the Mailbox delivers the message to the recipient by calling recipient.handle().
/**
* @notice Attempts to deliver `message` to the recipient.
* @param _metadata Arbitrary metadata that can be injected by a relayer,
* used by the ISM to verify the validity of `_message`.
* May include things like validator signatures, merkle proofs,
* zero-knowledge proofs of light client state, etc.
* @param _message Hyperlane formatted interchain message.
*/
function process(bytes calldata _metaData, bytes calldata _message) external;
See Message.solfor more details on Hyperlane message encoding.

Message lifecycle

The diagram below illustrates the lifecycle of an interchain message being sent from "Sender" to "Recipient", secured using a validator-based ISM.