We've recently launched a platform and we start aquiring emails from users, partly from facebook and partly on their own merit.
We want to start sending operational emails to our users, for example: users which completed or haven't completed certain actions in some interval.
I see that a lot of services do so, for example: twitter sends you emails when someone follows you or a weekly list of tweets, I've looked through the terms of facebook for example, but couldn't find any reference in their policy for such operational emails.
So the question is: what is the legality of such emails, how often can I do it, and how can I alleviate any legal problems by doing so. Of course we're going to implement an opt-out for emails.