Updates are only sent to a Buyer Group, which is a list of buyers who received your product by going through a cart Checkout, whether a regular paid checkout or a Free Checkout, and who also did not opt-out of joining your list during their checkout.
If you used the Seller Admin > "Send Free download link" feature to manually send links, there is a box you can check there to add a link for the recipient to join that product's Buyer Group. If the recipient did not click that link, or if you did not choose to send the link, then the recipient would not have been added to the Buyer Group list and would not receive any Updates.
In order to ensure that our post-checkout thank-you emails can get through, so buyers can obtain the downloads they pay for, it has been crucial to minimize any chance that E-junkie could ever be associated with any spam activity. As such, we only support voluntary opt-in mailing list building, where we presume the buyer does not wish to subscribe to anything nor receive any further emails from the seller unless the buyer deliberately takes an extra step to subscribe (e.g., by checking a box during checkout, or by clicking a link in the free-download email).