Sending mail with PHPMailer and Postmark error

<?php

ini_set('display_errors', '1');

if($_SERVER["REQUEST_METHOD"] == "POST") {
  $name = trim(filter_input(INPUT_POST, "user_name", FILTER_SANITIZE_STRING));
  $email = trim(filter_input(INPUT_POST, "user_email", FILTER_SANITIZE_EMAIL));
  $human = trim(filter_input(INPUT_POST, "user_human", FILTER_SANITIZE_STRING));
  $message = trim(filter_input(INPUT_POST, "user_message", FILTER_SANITIZE_STRING));

  // No input validation
  if($name == "" || $email == "" || $human == "" || $message == "") {
    echo "Please fill in the required fields: Name, Email, Are you human, Message";
    exit;
  }

 // Prevent HoneyPot Attacks
  if($_POST["check"] != "") {
    $error_message = "Form Input Incorrect";
    exit;
  }

  require("inc/PHPMailer/PHPMailerAutoload.php");

  $mail = new PHPMailer;

  // Invalid Email Validation
  if(!$mail->ValidateAddress($email)) {
    echo "Invalid Email Address";
    exit;
  }

  $email_body = "";
  $email_body .= "Name: " . $name ."\n";
  $email_body .= "Email: " . $email ."\n";
  $email_body .= "Message: " . $message ."\n";

  $mail->IsSMTP();
  $mail->SMTPAuth = true;
  $mail->SMTPDebug = 2;
  $mail->Host = "smtp.postmarkapp.com";
  $mail->Port = 26;
  $mail->Username = "#";
  $mail->Password = "#";

  $mail->setFrom($email, $name);
  $mail->addAddress("UP734253@myport.ac.uk", "James");
  $mail->isHTML(false);
  $mail->Subject = 'Message from ' . $name;
  $mail->Body    = $email_body;

  if(!$mail->send()) {
      echo 'Message could not be sent.';
      echo 'Mailer Error: ' . $mail->ErrorInfo;
      exit;
  }

  header("location:contactpage.php?status=thanks");
}