Trying to get contact form working

Expected behavior The form should send an email.

Actual behavior The email is always format invalid. I have attempted to follow the instructions via the Contact Form tutorial but always end up with the same error.

Resources (screenshots, code snippets etc.)

in action: www.webcatdesign.com.au

HTML CODE:

https://mdbootstrap.com/snippets/jquery/webcatdesign/1232275?action=forum_snippet

PHP in 'mail.php':

        <?php
$name = $_POST['name'];
$email = $_POST['email'];
$message = $_POST['message'];
$subject = $_POST['subject'];
header('Content-Type: application/json');
if ($name === ''){
print json_encode(array('message' => 'Name cannot be empty', 'code' => 0));
exit();
}
if ($email === ''){
print json_encode(array('message' => 'Email cannot be empty', 'code' => 0));
exit();
} else {
if (!filter_var($email, FILTER_VALIDATE_EMAIL)){
print json_encode(array('message' => 'Email format invalid.', 'code' => 0));
exit();
}
}
if ($subject === ''){
print json_encode(array('message' => 'Subject cannot be empty', 'code' => 0));
exit();
}
if ($message === ''){
print json_encode(array('message' => 'Message cannot be empty', 'code' => 0));
exit();
}
$content="From: $name \nEmail: $email \nMessage: $message";
$recipient = "webcat.design1@gmail.com";
$mailheader = "From: $email \r\n";
mail($recipient, $subject, $content, $mailheader) or die("Error!");
print json_encode(array('message' => 'Email successfully sent!', 'code' => 1));
exit();
?>