I've actually had this same experience with Postmark as a developer and user of the service. I told a couple of people I was using it instead of standing up my own server or whatever other solution and nobody has understood why. In terms of Postmark, having one less thing to have to worry about with my startup is great, especially when it's something as potentially hairy as sending email. But it's hard to get others to understand that, even when I'm not selling it.