I'm not an admin but if I understand correctly you are not allowed to email users using their billing emails. You should attach the file to the original game as well as the new project, otherwise you could ask players to provide proof of purchase.
Viewing post in Give exclusives to retro payments
Thanks for the reply, I'm probably going to Post a warning asking for the buyers (they aren't that many so it should be easy) to "comment" and give permission for me to send a key
This would be so much easier if there was an option to see the user that bought the game /not the email), I could easily send them a comment/link