hyperlane-monorepo/solidity/core/interfaces/IOutbox.sol

// SPDX-License-Identifier: MIT OR Apache-2.0
pragma solidity >=0.6.11;

import {IMailbox} from "./IMailbox.sol";

interface IOutbox is IMailbox {
    function dispatch(
        uint32 _destinationDomain,
        bytes32 _recipientAddress,
        bytes calldata _messageBody
    ) external returns (uint256);

    function cacheCheckpoint() external;

    function latestCheckpoint() external view returns (bytes32, uint256);

    function count() external returns (uint256);

    function fail() external;

    function cachedCheckpoints(bytes32) external view returns (uint256);

    function latestCachedCheckpoint()
        external
        view
        returns (bytes32 root, uint256 index);
}