# MailGun Webhook API Specification

## Overview

This document specifies the API for receiving webhook events from MailGun. The service logs all received event data to the console for monitoring and debugging purposes.

## API Endpoints

### POST /webhook

Receives webhook events from MailGun when email events occur.

#### Request Format

- **Method**: POST
- **Content-Type**: `application/x-www-form-urlencoded` or `multipart/form-data`
- **Headers**:
  - No custom headers required for initial implementation

#### Request Parameters

MailGun sends various parameters depending on the event type. Common parameters include:

**Event Identification:**
- `event` (string) - Type of event (delivered, failed, opened, clicked, bounced, complained, unsubscribed)
- `timestamp` (number) - Unix timestamp when the event occurred
- `token` (string) - Randomly generated string for message signature verification
- `signature` (string) - String with hexadecimal digits for signature verification

**Message Information:**
- `message-id` (string) - MailGun message ID
- `recipient` (string) - Email address of the recipient
- `domain` (string) - Domain from which the email was sent
- `Message-Id` (string) - SMTP Message-ID header

**Event-Specific Parameters:**

For **delivered** events:
- `message-headers` (string) - JSON string of message headers

For **failed** events:
- `severity` (string) - Severity level (temporary/permanent)
- `reason` (string) - Reason for failure
- `notification` (string) - Detailed notification message

For **opened** events:
- `city` (string) - City where email was opened
- `country` (string) - Country code
- `device-type` (string) - Device type (desktop/mobile/tablet)
- `client-os` (string) - Operating system
- `client-name` (string) - Email client name
- `ip` (string) - IP address

For **clicked** events:
- `url` (string) - URL that was clicked
- `city`, `country`, `device-type`, `client-os`, `client-name`, `ip` - Same as opened events

For **bounced** events:
- `code` (string) - SMTP error code
- `error` (string) - Detailed error message
- `notification` (string) - Bounce notification

For **complained** events:
- No additional parameters

For **unsubscribed** events:
- No additional parameters

#### Success Response

- **HTTP Status**: 200 OK
- **Content-Type**: `application/json`
- **Response Body**:
  ```json
  {
    "status": "received",
    "message": "Webhook event logged successfully"
  }
  ```

#### Error Responses

**Invalid Request (400 Bad Request)**:
- **Content-Type**: `application/json`
- **Response Body**:
  ```json
  {
    "error": "Invalid request format"
  }
  ```

**Server Error (500 Internal Server Error)**:
- **Content-Type**: `application/json`
- **Response Body**:
  ```json
  {
    "error": "Internal server error"
  }
  ```

## Execution Flow

1. **Receive webhook POST request** from MailGun
2. **Parse request body** (form-urlencoded or multipart data)
3. **Extract event data** from request parameters
4. **Log event data to console** with structured formatting:
   - Event type
   - Timestamp (both Unix and human-readable)
   - Recipient
   - All additional event-specific parameters
5. **Return success response** to MailGun

## Edge Cases

### Missing Event Type
- **Detection**: Check if `event` parameter is present
- **Handling**: Log warning and return 400 Bad Request

### Malformed Timestamp
- **Detection**: Check if `timestamp` can be parsed as number
- **Handling**: Log with current timestamp instead, continue processing

### Large Payload
- **Detection**: Monitor request body size
- **Handling**: Log truncated data if exceeds reasonable size

### Duplicate Events
- **Detection**: MailGun may send duplicate webhooks
- **Handling**: Log all events (no deduplication in initial implementation)

## Security Considerations

### Future Enhancements
For production deployment, consider:
- **Signature Verification**: Verify webhook authenticity using `timestamp`, `token`, and `signature`
- **IP Whitelisting**: Restrict to MailGun's IP ranges (implemented via CloudFlare)
- **Rate Limiting**: Prevent abuse

## Database Integration

- **Current Implementation**: No database operations required
- **Future Enhancement**: Store events in database for analysis

## Third-Party API Calls

- **Current Implementation**: No third-party API calls
- **Future Enhancement**: Could integrate with notification services

## Logging Format

Console output format:
```
========================================
MailGun Webhook Event Received
========================================
Event Type: delivered
Timestamp: 1234567890 (2024-01-01 12:00:00 UTC)
Recipient: user@example.com
Domain: mail.example.com
Message ID: <20240101120000.1.ABC123@mail.example.com>
----------------------------------------
Additional Parameters:
{
  "message-headers": "[...]",
  "token": "...",
  "signature": "..."
}
========================================
```

## Implementation Notes

- Use Express body-parser middleware for form data parsing
- All logging should use structured logger (debug package)
- Maintain type safety with TypeScript interfaces for event data
- Follow template's error handling patterns