def METHOD_NAME(self):
    djmail.outbox = []
    response = self.client.post(
        '/%s/%s/banktransfer/%s/%s/mail-invoice/' % (
            self.orga.slug, self.event.slug, self.order.code, self.order.secret),
        {'email': 'test@example.org'}
    )
    assert response.status_code == 302
    from django.contrib.messages import get_messages
    messages = list(get_messages(response.wsgi_request))
    assert len(messages) == 1
    assert str(messages[0]) == 'No pending bank transfer payment found. Maybe the order has been paid already?'
    assert len(djmail.outbox) == 0