Code for http://blog.pmg.co/integration-testing-swift-mailer/

CountableMemorySpool.php

final class CountableMemorySpool extends \Swift_MemorySpool implements \Countable
{
    public function count()
    {
        return count($this->messages);
    }

    public function getMessages()
    {
        return $this->messages;
    }
}

example.php

class Helloer
{
    private $mailer;

    public function __construct(\Swift_Mailer $mailer)
    {
        $this->mailer = $mailer;
    }

    public function sayHello($email)
    {
        $msg = \Swift_Message::newInstance()
            ->setSubject('Hello')
            ->setBody('Hello')
            ->setFrom('helle@example.com')
            ->setTo($email);

        $this->mailer->send($msg);
    }
}

class HelloerTest extends SwiftMailerTestCase
{
    private $helloer;

    public function testSayHelloSendsSpamEmails()
    {
        $this->helloer->sayHello('test@pmg.co');

        $this->assertMessageCount(1, 'should have sent one email');

        $msg = $this->getMessages()[0];

        $this->assertArrayHasKey('test@pmg.co', $msg->getTo());
    }

    protected function setUp()
    {
        parent::setUp();
        $this->helloer = new Helloer($this->mailer);
    }
}

SwiftMailerTestCase.php

abstract class SwiftMailerTestCase extends \PHPUnit_Framework_TestCase
{
    protected $mailer, $transport, $spool;

    protected function setUp()
    {
        $this->spool = new CountableMemorySpool();
        $this->transport = new \Swift_Transport_SpoolTransport(
            new \Swift_Events_SimpleEventDispatcher(),
            $this->spool
        );
        $this->mailer = new \Swift_Mailer($this->transport);
    }

    protected function assertMessageCount($count, $msg='')
    {
        $this->assertCount($count, $this->spool, $msg);
    }

    protected function getMessages()
    {
        return $this->spool->getMessages();
    }
}