Who are you allowed to email with a newsletter?
You may only send a newsletter to existing customers and to people who have given their unambiguous consent for this. Sending unsolicited emails to businesses or individuals (spamming) is a punishable offence in the Netherlands. The Netherlands Authority for Consumers & Markets (ACM) supervises compliance with this.
You can read about the legal basis on which you may process personal data and when you are allowed to contact people in Legal bases for email marketing.
In practice, you usually ask for consent using a subscribe form on your website. In your other communications (both online and offline), you can let people know that they can subscribe to your newsletter. In Laposta, you can design such a subscribe form and place it on your site.
When someone subscribes, they are added to your list immediately by default. If you want new subscribers to confirm their subscription by email first, read about when to use double opt-in.