Exception: Serialization of 'Closure' is not allowed

So I am not sure exactly what I would have to show you guys, how ever if you need more code please do not hesitate to ask:

So this method will set up the initMailer for Zend with in our application:

protected function _initMailer()
{
    if ('testing' !==  APPLICATION_ENV) {
        $this->bootstrap('Config');
        $options = $this->getOptions();
        $mail = new Zend_Application_Resource_Mail($options['mail']);
    }elseif ('testing'  ===  APPLICATION_ENV) {
        //change the mail transport only if dev or test
        if (APPLICATION_ENV <> 'production') {

            $callback = function()
            {
                return 'ZendMail_' . microtime(true) .'.tmp';
            };

            $mail = new Zend_Mail_Transport_File(
                array('path' => '/tmp/mail/',
                        'callback'=>$callback
                )
            );

            Zend_Mail::setDefaultTransport($mail);
        }
    }


    return $mail;
}

You can see the closure that lies with in. When I run any tests that use this code I get:

Exception: Serialization of 'Closure' is not allowed 

and thus all the tests in relation to this "closure" fails. So I am here asking you guys what I should do.

For clarification on the above, all were doing is saying that any email we send out we want to store information about that email in a folder in the /tmp/mail/ directory in a file.