PHP 电子邮件不起作用

我正在为一个网站创建一个电子邮件表单，html代码如下。

        <form class="form" name="htmlform" method="post" action="contact_process.php">
        <table width="450px">
        </tr>
        <tr>
         <td valign="top">
          <label for="first_name">First Name *</label>
         </td>
         <td valign="top">
          <input  type="text" name="first_name" maxlength="50" size="30">
         </td>
        </tr>
        <tr>
         <td valign="top"">
          <label for="last_name">Last Name *</label>
         </td>
         <td valign="top">
          <input  type="text" name="last_name" maxlength="50" size="30">
         </td>
        </tr>
        <tr>
         <td valign="top">
          <label for="email">Email Address *</label>
         </td>
         <td valign="top">
          <input  type="text" name="email" maxlength="80" size="30">
         </td>
        </tr>
        <tr>
         <td valign="top">
          <label for="telephone">Telephone Number</label>
         </td>
         <td valign="top">
          <input  type="text" name="telephone" maxlength="30" size="30">
         </td>
        </tr>
        <tr>
         <td valign="top">
          <label for="comments">Comments *</label>
         </td>
         <td valign="top">
          <textarea  name="comments" maxlength="1000" cols="25" rows="6"></textarea>
         </td>
        </tr>
        <tr>
         <td colspan="2" style="text-align:center">
          <input type="submit" value="Submit">
         </td>
        </tr>
        </table>
        </form>
        <p>* Mandatory fields</p>

尝试了很多下面的php代码（contact_process.php），但没有通过电子邮件将表单发送到指定的电子邮件。

<?php
if(isset($_POST['email'])) {
    $email_to = "johmwe@outlook.coM";
    $email_subject = "Feedback Form Submissions";

    function died($error) {
        // your error code can go here
        echo "We are very sorry, but there were error(s) found with the form you submitted. ";
        echo "These errors appear below.<br /><br />";
        echo $error."<br /><br />";
        echo "Please go back and fix these errors.<br /><br />";
        die();
    }
    // validation expected data exists
    if(!isset($_POST['first_name']) ||
        !isset($_POST['last_name']) ||
        !isset($_POST['email']) ||
        !isset($_POST['telephone']) ||
        !isset($_POST['comments'])) {
        died('We are sorry, but there appears to be a problem with the form you submitted.');       
    }
    $first_name = $_POST['first_name']; // required
    $last_name = $_POST['last_name']; // required
    $email_from = $_POST['email']; // required
    $telephone = $_POST['telephone']; // not required
    $comments = $_POST['comments']; // required
    $error_message = "";
    $email_exp = '/^[A-Za-z0-9._%-]+@[A-Za-z0-9.-]+'.[A-Za-z]{2,4}$/';
  if(!preg_match($email_exp,$email_from)) {
    $error_message .= 'The Email Address you entered does not appear to be valid.<br />';
  }
    $string_exp = "/^[A-Za-z .'-]+$/";
  if(!preg_match($string_exp,$first_name)) {
    $error_message .= 'The First Name you entered does not appear to be valid.<br />';
  }
  if(!preg_match($string_exp,$last_name)) {
    $error_message .= 'The Last Name you entered does not appear to be valid.<br />';
  }
  if(strlen($comments) < 2) {
    $error_message .= 'The Comments you entered do not appear to be valid.<br />';
  }
  if(strlen($error_message) > 0) {
    died($error_message);
  }
    $email_message = "Form details below.'n'n";
    function clean_string($string) {
      $bad = array("content-type","bcc:","to:","cc:","href");
      return str_replace($bad,"",$string);
    }
    $email_message .= "First Name: ".clean_string($first_name)."'n";
    $email_message .= "Last Name: ".clean_string($last_name)."'n";
    $email_message .= "Email: ".clean_string($email_from)."'n";
    $email_message .= "Telephone: ".clean_string($telephone)."'n";
    $email_message .= "Comments: ".clean_string($comments)."'n";

// creating email headers
$headers = 'From: '.$email_from."'r'n".
'Reply-To: '.$email_from."'r'n" .
'X-Mailer: PHP/' . phpversion();
@mail($email_to, $email_subject, $email_message, $headers);  
?>
Thank you for contacting us. We will be in touch with you very soon.
<?php
}
die();
?>

有人可以检查代码，我做错了什么。

可能的话，我想包括对发件人的自动回复和验证码图像，以解决安全问题。