Postmark do not allow bulk email, such as newsletters, and hence are not a comparable product.
We've got 5+ years of experience with our bulk email service, Newsberry, to back this up. Not only are our IP reputations carefully monitored and maintained, but we keep bulk and transactional as far away from each other as we can because they're treated SO differently by ISPs.
I was aware of Newsberry, but I still think that it is not comparable to SendGrid.
Even when we concentrate on bulk email, on one hand there is Newsberry, MailChimp, Campaign Monitor, Constant Contact, etc. And on the other are SendGrid, AuthSMTP, and now Amazon. The former provide an all inclusive solution: WYSIWG editing, templates, subscriber management, analytics, etc. while the latter are an order of magnitude cheaper, but offer a bare-bones solution that doesn't have much in the form of niceties and UI.
SendGrid is somewhere in the middle actually (and so is their pricing). They have analytics (and subscriber management if you wish) and even a crude UI where you can define email campaigns and newsletter templates (although clearly not their core business.) These feature put them above and beyond AuthSMTP which is basically an SMTP server. If Amazon has per campaign analytics (from my very brief look they do), then SendGrid should really be shacking in their boots, because Amazon has just undercut them big time without compromising on core competencies.