Mail function is not working

You are passing $email where there should be attachments. Look at wp_mail arguments.

Also you have not defined $to variable which in your case i assume should be $email.

Try this,

$name   = $_POST['cuf_sender'.$n];
    $email  = $_POST['cuf_email'.$n];
    $subject= $this->o['subpre'].' '.$_POST['cuf_subject'.$n];
    $msg    = $_POST['cuf_msg'.$n];

    $extra="";
    foreach ($_POST as $k => $f )
        if ( strpos( $k, 'cuf_field_') !== false )
            $extra .= $this->o[substr($k, 4, 7)].": $f\r\n";


    $headers =
    "MIME-Version: 1.0\r\n".
    "Reply-To: \"$name\" <$email>\r\n".
    "Content-Type: text/plain; charset=\"".get_settings('blog_charset')."\"\r\n";
    if ( !empty($from) )
        $headers .= "From: ".get_bloginfo('name')." - $name <$from>\r\n";
    else if ( !empty($email) )
        $headers .= "From: ".get_bloginfo('name')." - $name <$email>\r\n";

    $fullmsg =
    "Name: $name\r\n".
    "Email: $email\r\n".
    $extra."\r\n".
    'Subject: '.$_POST['cuf_subject'.$n]."\r\n\r\n".
    wordwrap($msg, 76, "\r\n")."\r\n\r\n".
    'Referer: '.$_SERVER['HTTP_REFERER']."\r\n".
    'Browser: '.$_SERVER['HTTP_USER_AGENT']."\r\n";

    if ( wp_mail( $email, $subject, $fullmsg, $headers) )
    {
        echo $to;
    exit();