When I sign up for an use an app I expect they will/could email me about the app. If they were interested before, they would probably like to hear about new fe

When I sign up for an use an app I expect they will/could email me about the app. If they were interested before, they would probably like to hear about new features and a free trial. If you don't have a lot of users now, just add a reply with stop if you would not like to receive emails from us. Track these manually till you come up with an opt out email system. Good luck converting some of the early signups to your app.