I have received a letter with a reminder that I have not sent in the statistics for a certain period.

If you have sent in the period in question, you will have received a confirmation of receipt by e-mail. Send this confirmation of receipt together with the reminder letter and a short report to ihinfo@cbs.nl. Statistics Netherlands can use the number on the confirmation of receipt to check what happened to the declaration. State your name and telephone number in the e-mail. Statistics Netherlands will then contact you for further processing.