PHP电子邮件表单代码不发送电子邮件

我不知道为什么这不会去任何地方。我设置它完全像这个线程的代码:发送电子邮件与PHP从html表单上提交相同的脚本

我更改了电子邮件地址，但它仍然没有发送给我。我错过什么了吗?提前感谢!

请帮忙!

<?php 
if(isset($_POST['submit'])){
$to = "email@example.com"; // this is your Email address
$from = $_POST['email']; // this is the sender's Email address
$first_name = $_POST['first_name'];
$last_name = $_POST['last_name'];
$subject = "Form submission";
$subject2 = "Copy of your form submission";
$message = $first_name . " " . $last_name . " wrote the following:" . "'n'n" . $_POST['message'];
$message2 = "Here is a copy of your message " . $first_name . "'n'n" . $_POST['message'];
$headers = "From:" . $from;
$headers2 = "From:" . $to;
mail($to,$subject,$message,$headers);
mail($from,$subject2,$message2,$headers2); // sends a copy of the message to the sender
echo "Mail Sent. Thank you " . $first_name . ", we will contact you shortly.";
// You can also use header('Location: thank_you.php'); to redirect to another page.
}
?>
<form action="" method="post">
First Name: <input type="text" name="first_name"><br>
Last Name: <input type="text" name="last_name"><br>
Email: <input type="text" name="email"><br>
Message:<br><textarea rows="5" name="message" cols="30"></textarea><br>
<input type="submit" name="submit" value="Submit">
</form>