At Fiesta we run our own custom mailserver. Since we handle a lot of incoming mail, we have to be confident our updates don’t break the mailserver. As can be expected, we have a large amount of test code and run it very often.
For those that haven’t had a chance to play with monkey patching, I hope this helps you come up with some clever, but clear (!) ways to test your code. It’s especially useful for testing methods that make library calls that need to exhibit less common/hard to force behavior.