$mailer->send(); calls the send() function on line 977 of the 'showkase/admin/plugins/contact/master/contactcore/class.phpmailer.php' file (PHPMailer). You would need to delve into the PHPMailer source code (or contact their own support) for more detailed information of exactly what the send() function does as we did not write this third-party library ourselves.
The $output variable from the 'contact.php' file is either the 'Optional Contact Form -> Success message' (if the email is sent successfully) or an appropriate error message (if the email is not sent successfully).
In the 'contact.php' file, try replacing:
$mailer->send();
$output = json_encode(array('type'=>'message', 'text' => $response));
echo($output);... with:
$mailer->send();
} catch (phpmailerException $exc) {
echo 'Mailer error: ' . $exc->errorMessage();If the email is not sent successfully, then an error message generated by PHPMailer itself should be displayed on the screen.
This might give you some indication of what the problem is.