Setting up Postmark with PHPMailer

<?php
 if ($_SERVER["REQUEST_METHOD"] == "POST") {
   $name = trim(filter_input(INPUT_POST, "name", FILTER_SANITIZE_STRING));
   $email = trim(filter_input(INPUT_POST, "email", FILTER_SANITIZE_EMAIL));
   $message = trim(filter_input(INPUT_POST, "message", FILTER_SANITIZE_SPECIAL_CHARS));

   if ($name == "" || $email == "" || $message == "") {
     $error_message = "Please fill out all the required fields: Name, Email, and Message";
   }
   if (!isset($error_message) && $_POST["address"] != "") {
     $error_message = "Bad form input";
    }
    require("../includes/phpmailer/PHPMailer-master/class.phpmailer.php");

    $mail = new PHPMailer;

    if (!isset($error_message) && !$mail->ValidateAddress($email)) {
      $error_message = "Invalid Email Address";
    }


    if (!isset($error_message)) {


    $email_body = "";
    $email_body .= $message;
    $mail->isSMTP();                                      // Set mailer to use SMTP
    $mail->Host = 'smtp.postmarkapp.com';  // Specify main and backup SMTP servers
    $mail->SMTPAuth = true;                               // Enable SMTP authentication
    $mail->Username = 'username';                 // SMTP username
    $mail->Password = 'password';                           // SMTP password
    $mail->SMTPSecure = 'tls';                            // Enable TLS encryption, `ssl` also accepted
    $mail->Port = 587;                                    // TCP port to connect to
    $mail->setFrom($email, $name);
    $mail->addAddress('email', 'name');
    $mail->isHTML(false);
    $mail->Subject = 'Comment on Website from ' . $name;
    $mail->Body = $email_body;
    if ($mail->send()) {
      header("location:index.php?status=thanks");
      exit;
    }
      $error_message = 'Message could not be sent.';
      $error_message .= 'Mailer Error: ' . $mail->ErrorInfo;
  }
}