PHP's mail() function with SendGrid' SMTPAPI

PHPmail.php

<?php
$headers   = array();
$headers[] = "MIME-Version: 1.0";
$headers[] = "Content-type: text/plain; charset=iso-8859-1";
$headers[] = "From: Sender Name <sender@domain.com>";
$headers[] = "Bcc: JJ Chong <bcc@domain2.com>";
$headers[] = "Reply-To: Recipient Name <receiver@domain3.com>";
$headers[] = "Subject: {$subject}";
$headers[] = "X-Mailer: PHP/".phpversion();
$headers[] = "X-SMTPAPI: {"category":"testing123"}";

mail($to, $subject, $email, implode("\r\n", $headers));
?>