You think that internal notifications should have notifications via email?
An optimal solution that some online casinos are already using is that their users can choose which type of messages they want to receive. The best way to do this in my opinion is that when you are registering on the site that it asks you there if you want to receive general newsletters to your e-mail box or not. Important notifications that concern your balance on the site, a possible investigation of your account, a successful deposit or withdraw to or from the casino and other stuff that is about your money should always be sent to your personal e-mail address also and not only via internal messages because some players including do a little gambling for a few days and then don't log in to any online casino for a few weeks.