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Errors & Responses

This section outlines the standard HTTP status codes and MX18-specific validation errors you might encounter while sending emails via the API or SMTP.

MX18 ensures all error responses are returned in a structured JSON format, making it easy to parse and debug.

Standard HTTP status codes

These codes conform to standard HTTP protocol behavior and apply to all API endpoints.

Status Code Meaning Description
400 Bad Request Malformed request. Required fields might be missing or incorrectly formatted.
401 Unauthorized API key is missing, expired, or invalid.
403 Forbidden The authenticated user does not have permission to perform this action.
404 Not Found The requested endpoint does not exist.
409 Conflict Duplicate request or conflicting resource creation.
422 Unprocessable Entity Validation failed. Common issues include missing recipient or subject.
429 Too Many Requests You've hit the rate limit. Wait or throttle your requests.
500 Internal Server Error Something went wrong on our end. Please try again later.
503 Service Unavailable MX18 is temporarily down for maintenance or facing downtime.

MX18-specific validation errors

These errors are returned as part of the JSON response and have an MX-specific code and human-readable message.

Error Code Message
MX-0001 Mandatory from email address is missing.
MX-0002 from email address is not valid.
MX-0003 Email subject is missing.
MX-0004 Email content is missing.
MX-0005 At least one recipient (to) must be specified.
MX-0006 Sender domain is invalid or not authorised.
MX-0007 Daily sending limit is over. Contact support.
MX-0008 Sending IPs are not configured for your account.

Sample error response

{
  "error": {
    "code": "MX-0005",
    "message": "At least one recipient (to) must be specified.",
    "status": 422
  }
}

You should always handle error responses by checking both the HTTP status code and the MX18-specific code for precise issue tracking.

Need help?

  • Make sure your request payload includes all required fields

  • Check your sender domain and IP configuration

  • Reach out to support@mx18.com for help with persistent errors