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Request returned status 400

private static $outlookApiUrl = "https://outlook.office.com/api/v2.0";

  public static function sendMail ($access_token,$user_email,$subject,$Content,$email){

    $arr= array(
        "Message" =>array(
       'Subject' => $subject,
      "Body"=>array(
          "Content-Type"=>"HTML",
          "Content"=>$Content,
        ),
      "ToRecipients"=>array(
        array(
            "EmailAddress"=>array(
                "Address"=>$email,
              )
          ),
        ),
    ));

    $json=json_encode($arr, true);

     $getMessagesUrl = self::$outlookApiUrl."/me/sendmail";


    return self::makeApiCall($access_token, $user_email, "POST",$getMessageUrl,$json);

 }

public static function makeApiCall($access_token, $user_email, $method, $url, $payload = NULL) {
      // Generate the list of headers to always send.
    $headers = array(
        "User-Agent: php-tutorial/1.0",
        "Authorization: Bearer ".$access_token,
        "Accept: application/json",
        "client-request-id: ".self::makeGuid(),
        "return-client-request-id: true",
        "X-AnchorMailbox: ".$user_email
        );

    $curl = curl_init($url);

    switch(strtoupper($method)) {
      case "POST":
      error_log("Doing POST");
      $headers[] = "Content-Type: application/json";
      curl_setopt($curl, CURLOPT_POST, true);
      curl_setopt($curl, CURLOPT_POSTFIELDS, $payload);
      break;
      default:
      error_log("INVALID METHOD: ".$method);
      exit;
    }

    curl_setopt($curl, CURLOPT_RETURNTRANSFER, true);
    curl_setopt($curl, CURLOPT_HTTPHEADER, $headers);
    $response = curl_exec($curl);
    error_log("curl_exec done.");

    $curl_errno = curl_errno($curl);
    $curl_err = curl_error($curl);
    if ($curl_errno) {
       //PRINT ERROR
    }
    else {
      error_log("Response: ".$response);
      curl_close($curl);
      return json_decode($response, true);
    }
  }

$send=OutlookService::sendMail($_SESSION["access_token"], $_SESSION["user_email"],"testing","<html><body>testing email.</body></html>","example@gmail.com");

    echo var_dump($send);