> ## Documentation Index
> Fetch the complete documentation index at: https://docs.hyperlane.xyz/llms.txt
> Use this file to discover all available pages before exploring further.

# Sequence Diagram

When a message is sent between chains, it goes through a verification process to ensure its authenticity. This page explains how that process works.

## Overview

The verification process involves three key components:

1. The **Mailbox contract** - The main entry point for message processing
2. The **ISM (Interchain Security Module)** - The security verification contract
3. The **Recipient contract** - The application that receives the message

## Sequence Diagram

<Note>
  * If the recipient does not implement `ISpecifiesInterchainSecurityModule` or `recipient.interchainSecurityModule()` returns `address(0)`, the default ISM configured on the [Mailbox](/docs/protocol/core/mailbox) will be used to verify the message.
  * This is omitted from the sequence diagram for clarity.
</Note>

```mermaid theme={null}
sequenceDiagram
    participant Relayer
    participant Mailbox
    participant ISM
    participant Recipient

    Relayer-->>ISM: moduleType()
    ISM-->>Relayer: ISM type
    Relayer->>Mailbox: process(metadata, message)
    Mailbox-->>Recipient: interchainSecurityModule()
    Recipient-->>Mailbox: ISM address
    Mailbox->>ISM: verify(metadata, message)
    ISM-->>Mailbox: true
    Mailbox->>Recipient: handle(origin, sender, body)
```

## How It Works

1. **Type Identification**

   * The Relayer calls `moduleType()` on the ISM to determine its category
   * Based on this type (e.g., `MULTISIG`, `ROUTING`, `AGGREGATION`), the Relayer knows what verification proof to collect
   * For Multisig ISMs, this means gathering cryptographic signatures from validators

2. **Message Submission**

   * The Relayer calls `Mailbox.process()` providing both:
     * The original message
     * The collected verification metadata

3. **Verification Process**

   * Mailbox queries the recipient contract via `interchainSecurityModule()`
   * If no ISM is specified, Mailbox uses the default ISM
   * Mailbox then forwards the message and metadata to the ISM's `verify()` function
   * The ISM executes its security logic (e.g., signature verification, threshold checking)

4. **Message Delivery**

   * Upon successful verification (when `verify()` returns `true`), Mailbox calls `recipient.handle()`
   * If verification fails, the transaction reverts and the message is rejected