PHPMailer SMTP Error: data not accepted

I'm trying out PHPMailer using outlook.com's SMTP server but I keep getting SMTP Error I followed the example code from PHPMailer's github page, and I've also looked at other questions on SO, but the answers there don't solve my problem

This is the code

<?php

date_default_timezone_set('Etc/UTC');

require_once 'vendor/autoload.php';
$mail = new PHPMailer;

$mail->SMTPOptions = array(
    'ssl' => array(
        'verify_peer' => false,
        'verify_peer_name' => false,
        'allow_self_signed' => true
    )
);

//Tell PHPMailer to use SMTP
$mail->isSMTP();
$mail->SMTPDebug = 2;

//Ask for HTML-friendly debug output
$mail->Debugoutput = 'html';

//Set the hostname of the mail server
$mail->Host = 'smtp-mail.outlook.com';
$mail->Port = 587;
$mail->SMTPSecure = 'tls';

//Whether to use SMTP authentication
$mail->SMTPAuth = true;

//Username to use for SMTP authentication
$mail->Username = "user@outlook.com";

//Password to use for SMTP authentication
$mail->Password = "pass";

//Set who the message is to be sent from
$mail->setFrom('user@outlook.com', 'User');

//Set who the message is to be sent to
$mail->addAddress('recipient@gmail.com', 'Recipient');

//Set the subject line
$mail->Subject = 'PHPMailer SMTP test';


$mail->Body    = 'This is the HTML message body <b>in bold!</b>';
//Replace the plain text body with one created manually
$mail->AltBody = 'This is a plain-text message body';

//send the message, check for errors
if (!$mail->send()) {
    echo "<br><br>Mailer Error: " . $mail->ErrorInfo;
} else {
    echo "Message sent!";
}

And here is the debug output

SERVER -> CLIENT: 220 BLU436-SMTP81.smtp.hotmail.com Microsoft ESMTP MAIL Service, Version: 8.0.9200.16384 ready at Mon, 20 Jun 2016 23:35:39 -0700 
CLIENT -> SERVER: EHLO localhost
SERVER -> CLIENT: 250-BLU436-SMTP81.smtp.hotmail.com Hello [139.193.110.46]250-TURN250-SIZE 41943040250-ETRN250-PIPELINING250-DSN250-ENHANCEDSTATUSCODES250-8bitmime250-VRFY250-TLS250-STARTTLS250 OK
CLIENT -> SERVER: STARTTLS
SERVER -> CLIENT: 220 2.0.0 SMTP server ready
CLIENT -> SERVER: EHLO localhost
SERVER -> CLIENT: 250-BLU436-SMTP81.smtp.hotmail.com Hello [139.193.110.46]250-TURN250-SIZE 41943040250-ETRN250-PIPELINING250-DSN250-ENHANCEDSTATUSCODES250-8bitmime250-VRFY250-AUTH LOGIN PLAIN XOAUTH2250 OK
CLIENT -> SERVER: AUTH LOGIN
SERVER -> CLIENT: 334 VXNlcm5hbWU6
CLIENT -> SERVER: xxx==
SERVER -> CLIENT: 334 UGFzc3dvcmQ6
CLIENT -> SERVER: xxx=
SERVER -> CLIENT: 235 2.7.0 Authentication succeeded
CLIENT -> SERVER: MAIL FROM:<user@outlook.com>
SERVER -> CLIENT: 250 2.1.0 user@outlook.com....Sender OK
CLIENT -> SERVER: RCPT TO:<Recipient@gmail.com>
SERVER -> CLIENT: 250 2.1.5 Recipient@gmail.com 
CLIENT -> SERVER: DATA
SERVER -> CLIENT: 354 Start mail input; end with <CRLF>.<CRLF>
CLIENT -> SERVER: Date: Tue, 21 Jun 2016 06:35:39 +0000
CLIENT -> SERVER: To: Recipient <Recipient@gmail.com>
CLIENT -> SERVER: From: User <user@outlook.com>
CLIENT -> SERVER: Subject: PHPMailer SMTP test
CLIENT -> SERVER: Message-ID: <405c2ef139a1fa30da7bd01a6f945eb0@localhost>
CLIENT -> SERVER: X-Mailer: PHPMailer 5.2.16 (https://github.com/PHPMailer/PHPMailer)
CLIENT -> SERVER: MIME-Version: 1.0
CLIENT -> SERVER: Content-Type: multipart/alternative;
CLIENT -> SERVER: boundary="b1_405c2ef139a1fa30da7bd01a6f945eb0"
CLIENT -> SERVER: Content-Transfer-Encoding: 8bit
CLIENT -> SERVER: 
CLIENT -> SERVER: This is a multi-part message in MIME format.
CLIENT -> SERVER: 
CLIENT -> SERVER: --b1_405c2ef139a1fa30da7bd01a6f945eb0
CLIENT -> SERVER: Content-Type: text/plain; charset=us-ascii
CLIENT -> SERVER: 
CLIENT -> SERVER: This is a plain-text message body
CLIENT -> SERVER: 
CLIENT -> SERVER: 
CLIENT -> SERVER: --b1_405c2ef139a1fa30da7bd01a6f945eb0
CLIENT -> SERVER: Content-Type: text/html; charset=us-ascii
CLIENT -> SERVER: 
CLIENT -> SERVER: This is the HTML message body <b>in bold!</b>
CLIENT -> SERVER: 
CLIENT -> SERVER: 
CLIENT -> SERVER: 
CLIENT -> SERVER: --b1_405c2ef139a1fa30da7bd01a6f945eb0--
CLIENT -> SERVER: 
CLIENT -> SERVER: .
SERVER -> CLIENT: 550 5.3.4 554-554 5.2.0 STOREDRV.Deliver; delivery result banner
SMTP ERROR: DATA END command failed: 550 5.3.4 554-554 5.2.0 STOREDRV.Deliver; delivery result banner
SMTP Error: data not accepted.