wordpress shortcode contactform

<?php
/*
Plugin Name: contact_shortcode
Plugin URI: http://nathanrobjohn.com
Description: contact form. Usage: <code>[contact email="your@email.address"]</code>
Version: 1.0
Author: Nathan Robjohn
Author URI:http://nathanrobjohn.com
*/


function nathan_shortcode_contact( $atts, $content = null)  {


    extract( shortcode_atts( array(
      'email' => get_bloginfo('admin_email')
      ), $atts ) );

      $content .= '<script type="text/javascript">
var $j = jQuery.noConflict();
$j(window).load(function(){
    $j("#contact-form").submit(function() {
      // validate and process form here
        var str = $j(this).serialize();
           $j.ajax({
           type: "POST",
           url: "' . get_template_directory_uri() . '/short/sendmail.php",
           data: str,
           success: function(msg){
                $j("#note").ajaxComplete(function(event, request, settings)
                {
                    if(msg == "OK") // Message Sent? Show the Thank You message and hide the form
                    {
                        result = "Your message has been sent. Thank you!";
                        $j("#fields").hide();
                    }
                    else
                    {
                        result = msg;
                    }
                    $j(this).html(result);
                });
            }
         });
        return false;
    });
});
</script>';


        // now we put all of the HTML for the form into a PHP string
        $content .= '<div id="post-a-comment" class="span12 contactform">';
            $content .= '<div id="fields">';
                $content .= '<h1>Enquire</h1>';
            $content .= '<div id="note"></div> <!--notification area used by jQuery/Ajax -->';
                $content .= '<form id="contact-form" action="">';
                    $content .= '<input name="to_email" type="hidden" id="to_email" value="' . $email . '"/>';
                    $content .= '<p class="span2">';
                        $content .= '<label class="error" for="name">Name *</label>';
                        $content .= '<input name="name" type="text" id="name"/>';
                    $content .= '</p>';
                    $content .= '<p class="span2">';
                        $content .= '<label for="email">Phone number *</label>';
                        $content .= '<input name="phone" type="text" id="phone"/>';
                    $content .= '</p>';
                    $content .= '<p class="span2">';
                        $content .= '<label for="subject">Best time to call</label>';
                    $content .= '<select name="time" id="subject" role="select" aria-required="true">
                                <option></option>
                                <option>Morning</option>
                                <option>Afternoon</option>
                                <option>Anytime</option>
                            </select>   ';
                    $content .= '</p>';
                    $content .= '<p class="span2">';
                        $content .= '<label for="email">E-mail address *</label>';
                        $content .= '<input name="email" type="text" id="email"/>';
                    $content .= '</p>';
                        $content .= '<p><label for="email">Enquiry message *</label></p>';
                    $content .= '<p class="span12"><textarea rows="16" cols="" name="message"></textarea></p>';
                    $content .= '<input type="submit" value="Submit" class="button" id="contact-submit" />';
                $content .= '</form>';
            $content .= '</div><!--end fields-->';
        $content .= '</div>';
    return $content;
}
add_shortcode('contact', 'nathan_shortcode_contact');

<?php
$plugin_name = 'contact_shortcode';


function ValidateEmail($email)
{
    /*
    (Name) Letters, Numbers, Dots, Hyphens and Underscores
    (@ sign)
    (Domain) (with possible subdomain(s) ).
    Contains only letters, numbers, dots and hyphens (up to 255 characters)
    (. sign)
    (Extension) Letters only (up to 10 (can be increased in the future) characters)
    */

    $regex = '/([a-z0-9_.-]+)'. # name

    '@'. # at

    '([a-z0-9.-]+){2,255}'. # domain & possibly subdomains

    '.'. # period

    '([a-z]+){2,10}/i'; # domain extension

    if($email == '') {
        return false;
    }
    else {
        $eregi = preg_replace($regex, '', $email);
    }

    return empty($eregi) ? true : false;
}

$post = (!empty($_POST)) ? true : false;

if($post)
{
    $name = stripslashes($_POST['name']);
    $time = stripslashes($_POST['time']);
    $phone = stripslashes($_POST['phone']);
    $subject = 'Contact Form';
    $email = trim($_POST['email']);
    $to = trim($_POST['to_email']);
    $message = stripslashes($_POST['message']);
    $error = '';

    // Check name

    if(!$name)
    {
        $error .= 'Please enter your name.<br />';
    }

    if(!$phone)
    {
        $error .= 'Please enter a phone number.<br />';
    }

    if(!$time)
    {
        $error .= 'Please select a time.<br />';
    }

    // Check email

    if(!$email)
    {
        $error .= 'Please enter an e-mail address.<br />';
    }

    if($email && !ValidateEmail($email))
    {
        $error .= 'Please enter a valid e-mail address.<br />';
    }

    // Check message (length)

    if(!$message || strlen($message) < 15)
    {
        $error .= "Please enter your message. It should have at least 15 characters.<br />";
    }

    if(!$error) // send email
    {
        $contents = "Name: $name \n\nEmail: $email \n\nSubject: $subject \n\nPhone: $phone \n\nTime: $time  \n\nMessage:\n $message";
        $mail = mail($to, $subject, $contents,
             'From: '.$name.' <'.$email.'>\r\n'
            .'Reply-To: '.$email.'\r\n'
            .'X-Mailer: PHP/' . phpversion());

        if($mail)
        {
            echo 'OK';
        }

    }
    else
    {
        echo '<div class="notification_error">'.$error.'</div>'; // set up error div for jQuery/Ajax
    }

}
?>