I tried sending without Ajax (turned of Javascript) and it seems to work. The returned success msg may be falsely returned though, check if you've gotten an email – if so,  as a a workaround you can turn off Ajax support on the main cforms config page.

The error you're seeing is a result from the first attempt to send you an email:

    if( @mail($to, $vsubject, stripslashes($message), $headers) )

Why it's causing the issue I can't say from here. Please check your form fields and setup and check your admin email settings for proper email formats. For whatever reason, your email server denies emailing the form data.

Do you have tracking enabled? Can you see the form submissions (&data) there?