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.
Steve doesn't take into account the amount of people that will actually make it through their search term entirely. Is there data somewhere on how often users click on suggestions as opposed to writing out the term in its entirety?
Even so, Google will still rank sites based on SEO and will in turn use that to suggest terms when users are typing. This just means that it's even more important to be the top result for a given search term.
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.