Dependency on mock method with PHPUnit

I'm attempting to write PHPUnit tests for an Email abstraction class i'm using. The class interacts with the Mailgun API but I don't want to touch this in my test, I just want to return the response I would expect from Mailgun.

Within my test I have a setup method:

class EmailTest extends PHPUnit_Framework_TestCase
{

    private $emailService;

    public function setUp()
    {
        $mailgun = $this->getMockBuilder('SlmMail\Service\MailgunService')
                        ->disableOriginalConstructor()
                        ->getMock();

        $mailgun->method('send')
                ->willReturn('<2342423@sandbox54533434.mailgun.org>');

        $this->emailService = new Email($mailgun);
        parent::setUp();
    }

    public function testEmailServiceCanSend()
    {
        $output = $this->emailService->send("me@test.com");
        var_dump($output);
    }
}

var_dump($output); is currently outputting NULL rather than the string i'm expecting. The method send i'm stubbing in the mock object has a dependency through an argument, and when I call $mailgun->send() directly it errors based on this so I wonder if this is what is failing behind the scenes. Is there a way to pass this argument in, or should I approach this a different way?