Here is a thumb rule for all start-ups, all app makers, mobile, Web or desktop: Just because you need my email address to create a user account does not give you the right to use this address for advertising.
It’s called permission marketing, people. When a user opens an account with your service or app, you need to ask for their permission for using their email information for marketing purposes. Make your intentions clear before using their information for your marketing. Don’t call them product updates, news or offers. Marketing is the right word.
No matter how great your product is, how good your intentions are, or how friendly your staff is; if you don’t follow this rule, people getting your marketing email will think “When did I subscribe to this?” and immediately unsubscribe.
You turn a potential opportunity into a negative experience. Will this user subscribe again? Realistically, chances are pretty low.