The newsletter module in Sitefinity is pretty basic, so I needed an enterprise-level newsletter software. I found a really good and inexpensive email marketing software called Newsletter Manager Pro. I liked it so much that I ended up integrating right into Sitefinity!
I made it into a module and you still have the ability to harvest contacts directly from your Sitefinity website since I created a subscription box control for it (no double entry needed). There is even an option to export subscribers from the "old" Newsletter Module in Sitefinity so you may import them into Newsletter Manager Pro!
Check out screenshots and info: