html - PHP email does not work

I am creating an email form for a website and the html code is as follows.

        


        































                
         

          First Name *
         
         

          
         
        
        
         

          Last Name *
         
         

          
         
        
        
         

          Email Address *
         
         

          
         
        
        
         


          Telephone Number
         
         

          
         
        
        
         

          Comments *
         
         

          
         
        
        
         

          
         
        
        
        
        
* Mandatory fields

Tried the php code (in contact_process.php) below a lot but does not email the form to the specified email.

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.

";
        echo $error."

";

        echo "Please go back and fix these errors.

";
        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.
';
  }
    $string_exp = "/^[A-Za-z .'-]+$/";
  if(!preg_match($string_exp,$first_name)) {
    $error_message .= 'The First Name you entered does not appear to be valid.
';
  }
  if(!preg_match($string_exp,$last_name)) {
    $error_message .= 'The Last Name you entered does not appear to be valid.
';

  }
  if(strlen($comments) < 2) {
    $error_message .= 'The Comments you entered do not appear to be valid.
';
  }
  if(strlen($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.


}
die();
?>

Can someone please check the code and what am I doing wrong.

Further more, I would like to include an auto respond to the sender and a CAPTCHA image if possible for security issues.