Routine spam on php contact form [duplicate]

I have a contact form on two different websites I have made for clients. At around 8-9pm everyday a blank message is sent using the contact form and straight to my clients' respective email addresses.

PHP:

<?php
$name = $_POST['full-name'];
$phone = $_POST['phone'];
$email = $_POST['email'];
$message = $_POST['message'];
$from = $_POST['full-name']; 
$to = 'mobileguitarworkshop@hotmail.com';

if(!empty($_POST['field'])) die();

$email_from = 'mobileguitarworkshop@hotmail.com';

$email_subject = "Enquiry from $name.\n";

$body = "From: $name.\n".
        "Email: $email.\n".
        "Message: $message.\n";

$headers = "From: $email \r\n";
$headers .= "Reply-To: $email \r\n";

mail($to, $email_subject, $body, $headers); 
header("Location: http://mobileguitarworkshop.co.uk/success.html");
exit();
?>

HTML:

<form action="contact.php" method="post" class="contact-form">

                    <label for="full-name">Name</label>
                    <input name="full-name" type="text" id="full-name" required>

                    <input type="text" id="field" name="field"/>

                    <label for="phone">Phone</label>
                    <input name="phone" type="tel" id="phone">

                    <label for="email">Email address</label>
                    <input name="email" type="text" id="email" required>

                    <label for="message">Message</label>
                    <textarea name="message" id="message"></textarea>

                    <input name="send" type="submit" value="SEND" id="sendBtn">
                </form>

I've tried adding 'required' to the Name and Email Address inputs to stop spammers, and also a hidden field that, if filled, directs them to 'success.html' without posting the message. If anyone can explain why this is happening that would be great. The hosting service I'm using is 1&1 IONOS. Thanks, Jack