SwiftMailer DKIM Sign

mail.php

<?php
require 'vendor/autoload.php';

$transport = Swift_SmtpTransport::newInstance('localhost', 25);
$mailer = Swift_Mailer::newInstance($transport);
// Read private key file
$privateKey = file_get_contents('/path/to/privatekey');
$domainName = 'example.com';
$selector = 'default';
$signer = new Swift_Signers_DKIMSigner($privateKey, $domainName, $selector);
// Create Message
$message = Swift_Message::newInstance();
// Attach signer
$message->attachSigner($signer);
$message
    ->setFrom(['[email protected]'])
    ->setTo(['[email protected]'])
    ->setSubject('Hello World')
    ->setBody('Hello World')
    ;
$result = $mailer->send($message);