"MethodNotAllowedError" / "POST is not allowed" - PHP woes

Hey Dreamweaver geniuses--

I've recently completed my first attempt at a "contact us" form using PHP.  When I test the form and click "submit",  I get this:

{"code":"MethodNotAllowedError","message":"POST is not allowed"}

My form code and PHP code are below.  Would appreciate any advice.  Oh--and does the webpage with the form need to be a PHP file?  Or can it be an HTML file that simply contains a form with a PHP action?  

********************************************

FORM CODE

<div id="contactBox">

  <form action="mailscript.php" method="post" enctype="multipart/form-data" name="contactForm" id="contactForm" title="Contact Form">

     <p><em>*Required field </em></p>

     <p>

       <input name="firstname" type="text" required class="label" id="firstname" placeholder="First Name*">

       <input name="lastname" type="text" required class="label" id="lastname" placeholder="Last Name*">

       <input name="email" type="email" required class="label" id="email" placeholder="Email address*">

       <input name="phone" type="text" required class="label" id="phone" placeholder="Phone*" />

       <select name="timezone" class="selectItem" id="timezone" title="Time Zone">

         <option>Time Zone</option>

         <option>Pacific</option>

         <option>Mountain</option>

         <option>Central</option>

         <option>Eastern</option>

         <option>Other</option>

       </select>

       <select name="besttime" class="selectItem" id="besttime">

         <option>Best Time To Call</option>

         <option>Morning</option>

         <option>Midday</option>

         <option>Afternoon</option>

         <option>Evening</option>

       </select>

       <select name="referral" required class="selectItem" id="referral">

         <option>How did you hear about us?</option>

         <option>Option 1</option>

         <option>Option 2</option>

         <option>Someone referred me</option>

       </select>

     </p>

      <input name="submit" type="submit" class="submitButton" id="submit" form="contactForm" formaction="mailscript.php" formenctype="multipart/form-data" formmethod="POST" title="Submit" value="Submit">

   </form>

</div>

*************************************

CODE FOR "mailscript.php"

<?php

$from="noreply@domain.com";

$email="info@domain.com";

$subject="SUBMISSION";

$firstname=$_POST['firstname'];

$lastname=$_POST['lastname'];

$their_email=$_POST['email'];

$phone=$_POST['phone'];

$timezone=$_POST['timezone'];

$besttime=$_POST['besttime'];

$referral=$_POST['referral'];

mail ($email, $subject, "From:".$from)

?>