I suggest calling the department of revenue, but don't use the contact information on the letter you received since there have been many fake letters that have been mailed. Go to your department of revenue's official website to get their phone number, and then call them to ask about the fraud letter you received and what you need to do regarding the letter.
You can post the name and state (if applicable) of the department of revenue listed on your letter, if you would like help searching for your department of revenue's contact information