[Gustomaximus]

While I would love to say 'just be reasonable' you should take some care here as you could be breaking the law by emailing people without permission. While laws differ by country you are best doing 3 things;

1) Include a check box saying you will mail people when they give you their email via sign-up. It's generally considered OK to pre-check this these days.

2) Distinguish promotional vs service emails you send. Its generally OK to send service emails without pre-consent e.g. new terms and conditions but you need permission to send promotional emails e.g. New refer a friend program.

3) Include an easy to find stop contacting me link on every communication. Don't try to hide this or make it difficult to use e.g. login in to update preferences. If people start marking you as spam this could hurt your ability to communicate with users once ISPs view your mail as lower quality.

Additionally I would avoid doing pop-up/notification style prodding. Any app that does this to me gets removed quick smart. Perhaps do this one time to update your current base but if you do this with any regularity any short term gains will likely come at a long term price.