Help With Constant Contact

I am unhappy with the current auto responder/ mass email program I am using. The problem is I have a list of several hundred people who have already opted in to my newsletter. I have read on activerain that many realtors love constant contact and I already receive emails on a daily basis from agents using constant contact. The question I have is will I have to get all my contacts to opt in again? Also would I be able to import all my contacts from outlook or a .csv file to their system?