PHP Mail not being sent to an outside email

Question

I have the following mail function which I am using to send an email. It works fine to the same email address that I am sending from. But when I change the email address of "$to" it is not being sent and doesn't give any error.

 <?php 

        if(!isset($_SESSION)) 
        { 
            session_start(); 
        }

    require 'PHPMailer/PHPMailerAutoload.php';

    $port = 587;
    $host = "mail.gmx.com";
    $security = 'tls';
    $username = "[email protected]";
    $password = "xxxxxx";

      function sendPaymentApprovalMailToPayee($to,$receiver,$payno,$amount){

global $port,$host,$security,$username,$password;
        $to = "[email protected]"; //When you change this to an outside email the mail doesn't get sent
        $from = "[email protected]";;
        $subject = "Payment Approved #".$payno;
        $message = "<p>Dear ".$receiver." ,</p>
    <p>We are sending this mail to confirm that the payment of Rs.".$amount." (Pay No : #".$payno." ) has been approved and successfully transferred to your account.</p>
    <p>Thanks & Best Regards,<br> Online Payment System - Team.</p>";

       $mail = new PHPMailer(); // create a new object
    $mail->IsSMTP(); // enable SMTP
    $mail->SMTPDebug = 1; // debugging: 1 = errors and messages, 2 = messages only
    $mail->SMTPAuth = true; // authentication enabled
    $mail->SMTPSecure = $security; // secure transfer enabled REQUIRED for GMail
    $mail->Host = $host;
    $mail->Port = $port; // or 587
    $mail->IsHTML(true);
    $mail->Username = $username;
    $mail->Password = $password;
    $mail->SetFrom($from);
    $mail->Subject = $subject;
    $mail->Body = $message;
    $mail->AddAddress($to);
     if(!$mail->Send())
        {
        //echo "Mailer Error: " . $mail->ErrorInfo;
        return false;
        }
        else
        {
            return true;
        //echo "Message has been sent";
        }



      }

      ?>

gmx mail settings

Server Settings

POP3 is an abbreviation for "Post Office Protocol Version 3". POP3 retrieves mails directly from a server into a mail program and deletes the mails on the server afterwards.

Incoming: Server: pop.gmx.com Port: 995 Encryption: SSL (If "SSL" is not available, you simply have to enable the "Encryption" function.)

Outgoing: Server: mail.gmx.com Port: 587 Encryption: STARTTLS If "STARTTLS" is not available, please use the protocol "TLS". If that option is not available, simply activate the option "Encryption".