When do you need to ask subscribers for permission again?
Who you may and may not email is governed by the Dutch Telecommunications Act (Telecommunicatiewet). You can read about the conditions set by this law and the legal bases under the GDPR in Legal bases for email marketing.
When do you need to ask for permission again?
Under the GDPR, you must be able to clearly demonstrate that the people you want to email have actually given permission for this. In the case of a customer relationship, you must be able to show when and how this person became a customer.
If you can look this up for each subscriber, you do not need to ask for permission again. Usually, one of these three situations applies:
Would you like to contact some of your subscribers again? Read Creating a re-permission campaign to learn how to set up such a campaign and record the permission given.