每次明确打开时，PHP文件都会发送邮件

I have a contact page which made with the help of PHPMailer. The minimal code is as follows

<form class="form-horizontal" method="POST" action="Mail.php" name="contactForm">
...
</form>

This works perfectly fine, but whenever I try to open Mail.php explicitly on the webserver, the webserver sends blank email every time. Here's a screenshot of email whenever I access Mail.php

Below is the minimal code from PHPMailer's Mail.php

$mail->Subject = $subject;
    $mail->Body    = "<b>From: </b>". $sender. "<br>" ." <b>Name: </b>". $yourName. "<br>". "<b> Message Body </b>" .$message;
    $mail->AltBody = "<b>From: </b>". $sender. "<br>" ." <b>Name: </b>". $yourName. "<br>". "<b> Message Body </b>" .$message;

        if(!$mail->send()) {
            echo 'Message could not be sent.';
            echo 'Mailer Error: ' . $mail->ErrorInfo;
        } 
        else {
            echo "Message has been sent....You're being redirected.....";
            }

How can I restrict the user to explicitly access Mail.php?

So, as suggested by Jiri Hrazdil and Ahmad Mobaraki, using isset should solve my problem, but I couldn't figure out how to implement that. So I came up with my own solution. I check, if the variables are empty in the first place, So this way if anyone directly tries to access the file it'll show a message the Fields are empty, This is the way I did it.

require 'PHPMailer/PHPMailerAutoload.php';
$yourName = $_POST['yourName'];
$sender  = $_POST['emailID'];
$subject  = $_POST['subject'];
$message  = $_POST['message'];
$to = 'email@email.com';
header('Refresh: 5; URL=whatever.php');

if(empty($yourName) || empty($sender) || empty($subject) || empty($message) || empty($message))
{
    echo "Fields are empty";
}
else
{
    //Working Code of PHP Mailer
}

You could add a condition to check, whether the values from the form are filled:

if (isset($_POST['sender'])) {
    // send mail
}

you should check if the request is POST in your php file for example:

if(isset($_POST['from'])){
   // send email 
}

when you try to open Mailer.php explicitly, your request is GET.

UPDATE:

from is not form! it is one of your form inputs! : <input name="from" type="text"> , you can use another field, like : name or message, when you try to open Mailer.php explicitly , none of from or to or name or subject or .... are filled with $_POST so there is no need to check all of them!

it is just enough to check for example to or from :

if(isset($_POST['to'])){
   // send email 
} else {
    die("forbidden! you can not open this file explicitly!")
}