Status Codes

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.
403	Forbidden	API key is valid but does not have permission for this action.	Use an API key with the required 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.
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.

In the Logs dashboard, filter by 4xx or 5xx to quickly find failed requests and debug integration issues.

Success (2xx)