Mailer will allow you to simply send mail to your activated users. It also has a simple opt-out system and will add an opt-out message at the bottom of the email.

It's a very simple no-frills addon, no HTML email, no list management, nothing fancy, just does what it says on the tin. Not to say more features won't find their way in at some point in the future but for now it's plain text email, opt-out and that's your lot!