Call from Google Map team for violation of term and asked to pay out of blue

We have been using Google Map API/service for years for free. Recently, we got an email saying we have been violated their term of use and asked to pay the OEM license. Google seems hired a team to hunt companies like us (ten of thousands) to collect the fee.

Did you get the same email? What is your response? We are thinking to switch to MapBox. Any other suggestion?