Mailscape has been a great resource for us at Facebook, it's low footprint, super simple web user interface and flexible reporting has saved us hours and hours of time mucking around in powershell. One of the killer features that people don't talk about much is that you can configure it out of the box to reflect emails through your system, back off of gmail's infrastructure and alert you on end to end message delays. No more hearing from your users that email is slow without knowing first!
