I've been working in email marketing for over 10 years, and list "health" is definitely one of the big challenges of email marketing.

Look for a service provider to help you both store, as well as collect emails. Depending on the site of the list you've collected so far, you might find a service provider like MailChimp or AWeber that can be very affordable.

Also, you may want to consider ways to keep these potential customers "warmed up" by launching a tease engagement campaign, giving early subscribers value via emails, and letting them know what they can expect once you launch. This will also give you a solid engagement metric on which to build forecasts once you launch.

If someone signs up to learn more, but doesn't hear from you for months, you are increasing the likelihood that person might have forgotten, switched email accounts, or may no longer be interested.

This is a little more than what you asked, but I thought relevant. Check out this article for related insights.
http://www.mediapost.com/publications/article/223901/tactics-to-reduce-email-address-churn.html

Feel free to schedule a follow up call and I'd be glad to dig in deeper and provide you with actionable suggestions.