First you better set debug to TRUE:
$email->SMTPDebug = true;
Or temporary change value of public $SMTPDebug = false; in PHPMailer class.
And then you can see the full log in the browser. For me it was too many emails per second:
...
SMTP -> FROM SERVER:XXX.XX.XX.X Ok
SMTP -> get_lines(): $data was ""
SMTP -> get_lines(): $str is "XXX.XX.XX.X Requested action not taken: too many emails per second "
SMTP -> get_lines(): $data is "XXX.XX.XX.X Requested action not taken: too many emails per second "
SMTP -> FROM SERVER:XXX.XX.XX.X Requested action not taken: too many emails per second
SMTP -> ERROR: DATA command not accepted from server: 550 5.7.0 Requested action not taken: too many emails per second
...
Thus I got to know what was the exact issue.