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You may receive a 403 error with error code 1010 (Access denied) when sending requests to the Resend API. This happens because requests without a User-Agent header are blocked before they reach the API.

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The Problem

The Resend API requires all HTTP requests to include a User-Agent header. When your HTTP client doesn’t send one, the request is rejected with a 403 status code and error 1010 before it ever reaches the API. This can be confusing because:
  • The error doesn’t come from the Resend API itself
  • Tools like curl automatically include a User-Agent header, so requests from the command line work fine
  • The error message doesn’t clearly indicate that the missing header is the cause

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How to Identify This Issue

  • You’re getting a 403 error, but your API key and domain are correct
  • The error response contains an error code of 1010
  • The same request works when using curl from the command line

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Solution

Add a User-Agent header to your HTTP requests. Example with Node.js fetch: 
fetch('https://api.resend.com/emails', {
  method: 'POST',
  headers: {
    'Authorization': 'Bearer re_123456789',
    'Content-Type': 'application/json',
    'User-Agent': 'my-app/1.0',
  },
  body: JSON.stringify({
    from: 'you@yourdomain.com',
    to: 'user@example.com',
    subject: 'Hello World',
    html: '<p>Hello World</p>',
  }),
});
If you’re using a Resend SDK, the User-Agent header is already included automatically. This issue typically only affects direct HTTP requests made without an SDK.

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Still Having Issues?

If you’ve added a User-Agent header and you’re still getting a 403 error:
  1. Verify that the header is being sent correctly by inspecting your outgoing request.
  2. Ensure your API key is valid and has the necessary permissions.
  3. Check that your domain is verified in the Resend Domains page.
  4. Contact Resend support with details about your error.