Edit: Didn’t have my coffee when I first read this. The post seems to be about user to user communication and not developer to user. Feel free to ignore this.
This is already a thing. If you go to “Edit Game” -> “Interact”, there will be an option to e-mail all people that have bought your game. Each e-mail has to go through admin review, in order to avoid spam.
Users already have the option to disable such e-mails from games in their settings.