def METHOD_NAME(self):
    testletter = Newsletter.objects.create(
        title="testletter",
        description="testdesc",
        sent=False,
    )
    self.client.post(
        reverse("newsletters:admin-send", args=[testletter.pk]), {"post": "yes"}
    )
    for message in mail.outbox:
        # If body is present then it is text/plain by default
        self.assertTrue(message.body)
        self.assertEqual(message.alternatives[0][1], "text/html")