Unit testing a side effecting actor with Akka

Given an actor with the following receive method definition:

def receive = {
  case SendEmail ⇒ {
    mailer.setup
    val body = prepareMessage(config.getString("source"), config.getString("template"))
    mailer.send(body, System.getenv("mailuser"), List(""), None, "stuff")
    listener.foreach(_ ! SendEmail)
    log.info("hey") //This line does get executed.
    self ! PoisonPill
  }
}

and a test class (Written with Scalacheck.) with the definition:

class EmailActorSpec extends ActorSpec {
  behavior of "An EmailActor"
  it should "send an email" in {
    val mailMock = new TestMailAccessor
    val props = Props(new EmailActor(Some(testActor), mailMock))
    val emailer = system.actorOf(props, "EmailActor")
    emailer ! SendEmail
    val result = expectMsgType[SendEmail](100 millis)
    expectMsg(result)
  }
}

Note that ActorSpec mixes in the following trait:

trait StopSystemAfterAll extends BeforeAndAfterAll {
    this: TestKit with Suite ⇒
    override protected  def afterAll() {
        super.afterAll()
        system.shutdown()
    }

I end up with the following error:

[INFO] [01/26/2015 13:49:27.287] [testsystem-akka.actor.default-dispatcher-4] [akka://testsystem/user/EmailActor] Message [akka.actor.PoisonPill$] from Actor[akka://testsystem/user/EmailActor#1885968450] to Actor[akka://testsystem/user/EmailActor#1885968450] was not delivered. [1] dead letters encountered. This logging can be turned off or adjusted with configuration settings 'akka.log-dead-letters' and 'akka.log-dead-letters-during-shutdown'.

How do I setup the tests so that when testing for message reception they actually pass?