The mass notifications seem to be a clear sign that Google is getting ready to launch a new mobile ranking algorithm. Preceding this letter, Google sent out some other signals that this algorithm change would be happening. It recently launched a mobile friendly testing tool and mobile usability reports in Google Webmaster Tools. It also started showing mobile-friendly labels in the search results. It seems something big is happening.
What should you do? If you received this notification, talk to your web developer about making your website mobile friendly. If you are not sure, even the more reason to reach out. If you don’t have a relationship with a web developer, you better start one. Something big is happening. You don’t want to be hit after what appears to be a new pending algorithm is launched.