Since you don't say a lot about what your business is about and what your goals are, I can say there are three e-mail list management programs I recommend:
1. Constant Contact: Affordable, easy-to-use and customer support is excellent.
2. Aweber: Manages your list and offers auto-responder technology, a bit of a steeper learning curve, but the support is excellent.
3. 1shoppingcart.com: If you plan to sell products online, this is the best choice.
4) InfusionSoft: Quite pricey but very robust.