explain email injection

PHP
<?php
if(isset($_POST['name']))
{
$name = $_POST['name'];
$replyto = $_POST['replyTo'];
$message = $_POST['message'];
$to = 'root@localhost';
$subject = 'My Subject';
// Set SMTP headers
$headers = "From: $name \n" .
"Reply-To: $replyto";
mail($to, $subject, $message, $headers);
}
?>

Source

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