The Postmark team is absolutely awesome. Forrst has been a happy customer since very early on, and Chris and team have gone out of their way to be helpful wherever possible. It's a huge reason why we haven't even considered Amazon or anyone else.
I second this. We've been sending out daily e-mail reports with PDF attachments to our customers and their API is easy to setup, easy to use and even makes attachments simple. To me, they've picked one thing and they're owning it.