1. You can an email in your regular email stating that there is a new message in your Berichteninbox. (No clickable link, presumably to avoid phishing.)
2. You go to mijn.overheid.nl to access your Berichteninbox. You sign in with DigID.
3. You open the mentioned message, which says a PDF with the actual letter is attached.
4. You open the PDF.
5. The PDF says you'll be able to file your tax returns a month from now.