MM Forms plugin error

I am trying to figure out why I keep getting an error when I submit a form that I have on my blog.

The error says: “Failed to send your message. Please try later or contact administrator by other way.”

I looked through the code and I found where this error occurs but I’m not sure WHY it is throwing the error. The section of the code where I found this looks like this:

function message($status) {
switch ($status) {
case ‘mail_sent_ok’:
return __(‘Your message was sent successfully. Thanks.’, ‘mmf’);
case ‘mail_data_saved’:
return __(‘Form data saved successfully. Thanks for submitting!’, ‘mmf’);
case ‘mail_data_not_saved’:
return __(‘Error occured in saving form data.’, ‘mmf’);
case ‘mail_sent_ng’:
return __(‘Failed to send your message. Please try later or contact administrator by other way.’, ‘mmf’);
case ‘validation_error’:
return __(‘Validation errors occurred. Please confirm the fields and submit it again.’, ‘mmf’);
case ‘accept_terms’:
return __(‘Please accept the terms to proceed.’, ‘mmf’);
case ‘invalid_email’:
return __(‘Email address seems invalid.’, ‘mmf’);
case ‘invalid_required’:
return __(‘Please fill the required field.’, ‘mmf’);
case ‘captcha_not_match’:
return __(‘Your entered code is incorrect.’, ‘mmf’);
}
}

The part that I am confused about is what ‘mail_sent_ng’ means. If anyone can help me out it would be greatly appreciated!

Rachael