@Override
	protected Response receive() throws IOException {
		final Response response = super.receive();
		if (response.getCode() == 421) {
			serverClosingTransmissionChannel = true;
		}
		return response;
	}
Returns the server response. It takes note of a 421 response code, so QUIT
will not be issued unnecessarily.
src/test/java/de/gessnerfl/fakesmtp/smtp/client/SmartClient.java