mail.send is not accepting HTML/ CSS code

I have copied the (HTML) source code from a received email just to have the template/format of that email, and this is the error message that I am receiveing when running the API:

Internal : Unexpected error Error during Web API HTTP Request
HTTP Status Code: 400
HTTP Response Content: {
  "error": {
    "code": "BadRequest",
    "message": "Unable to read JSON request payload. Please ensure Content-Type header is set and payload is of valid JSON format.",
    "innerError": {
      "date": "2020-12-02T17:01:05",
      "request-id": "3ab50fc1-0c13-4536-96db-bf6b9b7a736d",
      "client-request-id": "3ab50fc1-0c13-4536-96db-bf6b9b7a736d"
    }
  }
}

I have tested basic commands using HTML such as making text bold and line breaks which work, but anything more complex and I will receive an error.

Update** Thank you for your response guys, the call that I'm using looks like this:

{
  "message": {
    "subject": "[Subject]",
    "body": {
      "contentType": "HTML",
      "content": "[Email Body Content]"
    },
    "toRecipients": [
      {
        "emailAddress": {
          "address": "[Email Address]"
        }
      }
    ],
    "internetMessageHeaders":[
      {
        "name":"x-custom-header-group-name",
        "value":"Nevada"
      },
      {
        "name":"x-custom-header-group-id",
        "value":"NV001"
      }
    ]
  }
}

**Another update ** Upon changing the content type to text/html is recieved this error:

Internal : Unexpected error Error during Web API HTTP Request
HTTP Status Code: 415
HTTP Response Content: {"error":{"code":"RequestBodyRead","message":"A supported MIME type could not be found that matches the content type of the response. None of the supported type(s) 'Microsoft.OData.ODataMediaType, Microsoft.OData.ODataMediaTyp...' matches the content type 'text/plain; charset=utf-8'."}}

note i also have tried text/plain as the content type.

Thank you