SMTP Error: The following recipients failed:XXXX

I've encountered the same problem. Managed too fix it when i commented the next row:

 $mail->isSMTP(); 

Noticed you already found an answer, however maybe this will fix the problem for other people.

This does prevent using your external SMTP server as RozzA stated in the comments.

Maybe your class.phpmailer.php file is corrupt. Download the latest version from : https://github.com/PHPMailer/PHPMailer

$mail->SMTPDebug  = 1; // enables SMTP debug information (for testing)
                               // 1 = errors and messages
                               // 2 = messages only

try inlcuding this

$mail->SMTPDebug  = 1;