Obviously when you're at the scale of 37Signals... you're gonna roll this stuff yourself. But! I don't want people here to get discouraged about using 3rd party services. I too am guilty of "NIH syndrome" or the need to reinvent the wheel all the time. I've setup mail infrastructure. It's not pretty, and it's actually not necessary any more.

I've been using Postmark for a while now and it's been fantastic. They provide a great admin interface with insight into what is being sent and why mail might not be getting delivered. The setup process is very simple (none of the downsides 37signals mentions... ) you essentially add some SPF and DKIM DNS entries and you're off to the races.