Do not comply with such non-formal requests. If in doubt, please ask the Villingen Schwenningen tax office.
When acquiring a property, you will usually receive a notice of land transfer tax which includes a request for payment. Other requests for payment are only made if a tax assessment notice has been issued beforehand. Formal reminders or enforcement notices can only be issued if such requests for payment have not been complied with. In case of doubt, you should also check the bank details stated on such a letter with the applicable bank details of the tax office. You can find the bank details on the homepage of the tax office or on a current tax assessment notice.