Make start_ and stop_smtp_daemon tear down also on failing tests
At the moment, tests that require a smtp daemon start and stop that daemon at the beginning and the end of the test case.
This is a) cumbersome and b) it has the disadvantage that if an expection fails, rspec aborts the further execution of the test case and hence the smtp daemon is never stopped and the next test execution fails, because a new smtp daemon can't bind on the testing port, as the previous one is still listening there.
I guess, the plumbing and boilerplate for these kinds of tests could be improved with onboard mechanisms of rspec.