In order to show the exact error returned by the SMTP server please do so:
Open the following file in your site root:
/dmxConnectLib/PHPMailer/PHPMailer/PHPMailer.php
and edit line 401 from:
public $SMTPDebug = 1;
to
public $SMTPDebug = 3;
save the file and try sending the mail once again. You will see more detailed error in the console - please paste it here.