since the form is already built, you need to use universal Email
sending the newsletter though wont happen on the signup up page.
you would create an admin page that you using for creating the news letter using an HTML Editor instance.
Your friends over here at WebAssist! These Dreamweaver extensions will assist you in building unlimited, custom websites.
These out-of-the-box solutions provide you proven, tested applications that can be up and running now. Build a store, a gallery, or a web-based email solution.