When you are receiving tax letters you are notified per email. You can check any tax letter in your vatjar account, in the Tax letters menu.
One of the most frequent type of tax letters is the penalty tax letter. The reason for receiving this type of letter is either exceeding the payment deadline or late submission of the VAT return.

If the reason is late submission of the VAT return, please check with your Account Manager to find the exact reason for this. 

If the reason is a late payment of the VAT return, exceeding the deadline, then you should pay as soon as possible the penalties, some of the letters also state a deadline for the penalty payment, e.g. "pay the xx amount within 7 days".

You can always check the recommended action, under the "Translated action" section, by clicking on a tax letter. You can read more about this menu here.