I'm a huge fan of MailChimp. The 1,000 subscribers forever free plan is awesome for small businesses and the monthly fee is affordable for larger lists.
I've used a few before (Interspire, Constant Contact, Bronto) and have found MailChimp to be the most powerful and up to date on the latest technologies and best practices. The interface is intuitive as well and templates are pretty easy to build. I'm not a programmer but when I've had to work with its API our developers have always had an easy time with it.
That said, I've never tried Postmark. I think most will let you sign up and at least try it out for free, so I would suggest doing that and finding which one works best for you.