We only maintain Buyer Group lists of the actual buyers for each product in your Seller Admin. We can import an existing list of customers to any product's Buyer Group for you, and when you send a free download link manually you can also include a link inviting the recipient to join that product's list, but otherwise we have no way to add individual emails independently of actual purchases.
Our email Newsletters/Updates feature is really just a very basic, simple extension of the thank-you emails we send to buyers after their payment is completed, and intended primarily to accommodate only occasional and sporadic mass-email needs. This is also our only service not included in your monthly E-junkie subscription fee, requiring payment of a nominal surcharge to cover the spike in overhead it incurs; see this help page for details and pricing:
If you expect to send newsletters on any sort of regular basis or need more sophisticated features such as scheduling mailings, allowing people to subscribe without a purchase, etc., you would probably find a dedicated mailing list specialist service such as Aweber to be considerably more useful and possibly cheaper than our a la carte newsletter fees. You could use our built-in Aweber integration method to have us submit opted-in buyers to Aweber for subscription to your list(s) there:
For other mailing list services, if they can accept PayPal IPN to subscribe new buyers, they may be able to accept our IPN-style custom/3rd-party Integration submissions: