After a webinar on email marketing, I wrote an overview of Dutch anti-spam rules. I referred to OPTA, an old fine and several practical exceptions. For current decisions, I would use the ACM's explanation; it is the present authority.
Consent is the starting point
For commercial electronic messages, the general rule is to send them to people who consciously agreed beforehand. Explain what they are agreeing to and keep evidence of that choice. A pre-ticked box or a clause buried in terms does not meet that standard. Make the sender clear as well.
The ACM lists an exception for existing customers: you may message them about your own products or services related to a previous purchase. This does not give you permission to advertise to every contact-form user or survey respondent. Every message needs a fast, free way to unsubscribe.
Partners and forwarding
Take care with addresses acquired through a partner or affiliate. Establish who obtained consent, for which sender and content. Passing on a list does not automatically transfer permission. The ACM also sets conditions for tell-a-friend features: people must choose to forward without a reward, and recipients' personal data must not be retained for later marketing.
My old article got the principle of consent right, but used outdated authority names and overly broad wording. Before a campaign, check current ACM guidance and document your own signup process.