The error message you're encountering, "Incorrect Content-Type: must be textual to extract_string, JSON to extract_json," suggests that there may be an issue with the format or content type of the data being sent or received by the server during the process of creating a new mailbox in SmarterMail.

Here are steps you can take to troubleshoot and potentially resolve this issue:

1. Check API Endpoint:

   • Ensure that you are using the correct API endpoint for creating a new mailbox in SmarterMail.

2. Verify Request Payload:

   • Review the request payload to ensure it is correctly formatted and includes all required parameters. Check for any discrepancies in the JSON data.

3. Check Content-Type Header:

   • Verify that the Content-Type header in the request is set to application/json. This header tells the server that the request body is in JSON format.

4. Validate JSON Format:

   • Ensure that the JSON data being sent is valid and properly formatted. You can use online JSON validators to verify the correctness of your JSON payload.

5. Inspect Response:

   • Examine the response received from the SmarterMail server. It may contain additional information about the error, which can help pinpoint the issue.

6. Check for Server-Side Errors:

   • Review server logs or error messages for any indications of server-side issues that might be causing this problem.

7. Update SmarterMail:

   • Ensure that you are using a supported and up-to-date version of SmarterMail. If not, consider updating to the latest version, as it may contain bug fixes or improvements related to API interactions.

8. Test with a Different Client:

   • If you're using a third-party application to send the API request, try using a different client (e.g., Postman) to rule out any client-specific issues.

9. Contact SmarterMail Support:

   • If the issue persists and you've exhausted all troubleshooting steps, consider reaching out to SmarterMail support or their community forums for specific assistance related to API integration.

10. Review API Documentation:

    • Go through SmarterMail's API documentation to ensure you're using the correct endpoints, headers, and parameters for creating a new mailbox.

Keep in mind that this error is specific to the interaction with SmarterMail's API, so it's important to focus on the API request, payload, and response handling in your integration.