PHP主題電子郵件似乎打破了接收聯繫表

Question

我寫了一個聯繫表，當我輸入'姓名，電子郵件和消息'我收到電子郵件。但是，當我輸入主題時，它會將我帶到「謝謝」頁面，但我似乎從未收到該電子郵件。

我試過在PHP腳本中更改不同的變量，但這不起作用。我無法確定阻止電子郵件發送的因素。見下面的代碼。

<?php 

error_reporting(-1); 
ini_set('display_errors', 'On'); 
set_error_handler("var_dump"); 

if(isset($_POST['submit'])){ 
$first_name = $_POST['first_name']; 
$last_name = $_POST['last_name']; 
$email = $_POST['email']; 
$subject = $_POST['subject']; 
$message = $_POST['message']; 
$formcontent = "From: $first_name $last_name \n \n Message: $message"; 
$to = 'EMAIL@ADDRESS.DONTSHOW'; 
$mailheader = "From: $email \r\n"; 
mail($to, $subject, $formcontent, $mailheader) or die("Error!"); 
echo "Thank You! Your kind message has been recieved!"; 
} 
?> 

<form method="POST" action="mail.php"> 
        <div class="form-inline"> 
         <label for="Firstname">Name</label> 
         <input type="text" class="form-control" 
         id="Firstname" name="first_name" placeholder="First Name"> 
         <input type="text" class="form-control" 
         id="Lastname" name="last_name" placeholder="Last Name"> 
        </div>   
        <div class="form-group"> 
         <label for="Email">E-mail</label> 
         <input type="email" class="form-control" 
         id="Email" name="email" placeholder="Email Address"> 
        </div> 
        <div class="form-group"> 
         <label for="Subject">Subject</label> 
         <input type="text" class="form-control" 
         id="Subject" name="subject" placeholder="Let me know what it's about"> 
        </div> 
        <div class="form-group"> 
         <label for="Message">Message 
         </label> 
         <textarea class="form-control" 
         id="Message" name="message" rows="6" placeholder="Send me something awesome!"></textarea> 
        </div> 
        <button type="submit" name="submit" class="btn btn-default">Submit 
        </button>    
       </form>