The General Data Protection Regulation (GDPR) and is enforceable as law in all EU member states on May 25, 2018. While Mailster is technically compliance with this law you may have to bring some additional thing in place to make the whole process compliance for your site.
Many countries have strict laws when it comes to email marketing which is a good thing. With Mailster you have a tool you can fulfill legal requirements from almost any country on the world. Some countries require you to add a confirmation checkbox where users have to agree that they sign up to your service.