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

# Status Codes

> Reference for the HTTP status codes the Lettr API returns, covering 2xx success, 4xx client errors, and 5xx server errors with fixes.

The Lettr API uses standard HTTP status codes in every response. When viewing [API Logs](https://app.lettr.com/logs), you can filter by status code to quickly find successful requests or errors.

## Success (2xx)

| Code    | Name       | When you'll see it                                                                                                                       |
| ------- | ---------- | ---------------------------------------------------------------------------------------------------------------------------------------- |
| **200** | OK         | Successful GET, PATCH, or DELETE requests. The response body contains the requested or updated resource.                                 |
| **201** | Created    | Successful POST requests that create a resource (e.g. sending an email, creating a domain). The response body contains the new resource. |
| **204** | No Content | Successful DELETE requests with no response body.                                                                                        |

## Client errors (4xx)

These indicate a problem with the request (missing or invalid API key, bad parameters, or missing resource).

| Code    | Name                          | Cause                                                                                       | What to do                                                                                                                  |
| ------- | ----------------------------- | ------------------------------------------------------------------------------------------- | --------------------------------------------------------------------------------------------------------------------------- |
| **400** | Bad Request                   | Invalid request format, malformed JSON, or invalid parameters.                              | Check the response body for an error message and fix your request.                                                          |
| **401** | Unauthorized                  | Missing API key or invalid API key.                                                         | Ensure the `Authorization: Bearer <api_key>` header is set with a valid [API key](/learn/api-keys/introduction).            |
| **403** | Forbidden                     | API key is valid but does not have permission for this action.                              | Use an API key with the required [permissions](/learn/api-keys/permissions) or create a new key.                            |
| **404** | Not Found                     | The requested resource does not exist (e.g. wrong domain name, email ID, or template slug). | Verify the ID or identifier in the URL path.                                                                                |
| **409** | Conflict                      | Resource already exists or state conflict (e.g. domain already added).                      | Check the response message; you may need to use a different identifier or update the existing resource.                     |
| **422** | Unprocessable Content         | Request body failed validation (e.g. invalid email address, missing required field).        | The response includes an `errors` object with field-level messages. Fix the listed fields and retry.                        |
| **429** | Too Many Requests             | Rate limit exceeded.                                                                        | Retry after the time indicated in the `Retry-After` header. See [rate limits](/knowledge-base/troubleshooting/rate-limits). |
| **451** | Unavailable For Legal Reasons | Request blocked for legal or compliance reasons.                                            | Contact support if you believe this is in error.                                                                            |

## Server errors (5xx)

These indicate a temporary problem on Lettr's side. Retrying later usually succeeds.

| Code    | Name                  | When you'll see it                                                |
| ------- | --------------------- | ----------------------------------------------------------------- |
| **500** | Internal Server Error | An unexpected server error occurred.                              |
| **502** | Bad Gateway           | The server received an invalid response from an upstream service. |
| **503** | Service Unavailable   | The service is temporarily overloaded or in maintenance.          |

<Tip>
  In the [Logs](https://app.lettr.com/logs) dashboard, filter by 4xx or 5xx to quickly find failed requests and debug integration issues.
</Tip>

## Next steps

<CardGroup cols={2}>
  <Card title="API Logs" icon="list" href="/learn/logs/introduction">
    Filter, search, and export API logs
  </Card>

  <Card title="Troubleshooting" icon="wrench" href="/knowledge-base/troubleshooting/authentication">
    Fix common authentication and delivery issues
  </Card>
</CardGroup>