At its current iteration, it acts as a layer between your application and your SMTP server, neither of which we provide, but it has a strong backend UI, a great API, and a very robust template system that users love.
In the coming weeks, PostageApp is rolling out a hosted SMTP service, which means that it will become a great end-to-end solution for users like yourself looking to send emails through their application.
However, I do recommend that you give each of the services a try, there are many, just so you know what you and your application needs. (For example, some services do not let you have multiple addresses in an API call.) Would be very happy to chat if you have any questions, so just send me an email at email@example.com if you do!
Hope that helps.
(Full Disclosure: I am the Product Manager of PostageApp)