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.
So glad you're happy with Postmark and thanks for the kudos - the entire team really appreciates it.
37S is doing a great job with all of their techniques. Of course, they also have an incredible sending volume on their side that helps improve the impact of any of these techniques.
Postmark's inbound email mail parsing feature is very nice. I used it last week to set up automated order shipped confirmation emails for an e-commerce site we built. I just set up an auto forward to the Postmark inbound address, and they turn around and pass a nicely parsed JSON representation of the email to a URL on our web server.
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.