It's difficult to give you precise instructions without knowing more about who your email provider is and whether or not you are using the email client that came with your device or downloaded another email app.
Basically, you need to open the Gransnet email in your Spam folder then look in one of the top corners of the app for a drop-down menu (on my Apple and Android devices using Gmail I need to click 3 little dots), on the menu you should be able to click 'Not Spam'. There may even be a button saying 'Not Spam' somewhere too. This will tell your email provider that emails from this particular address are not to be marked as Spam in future.
You can also add the Gransnet news letter email address to your contacts list; [email protected], this will also tell your provider not to mark it as Spam.