I've gotten a ton of these emails recently, seems Google is changing how their ReCaptcha API works. The Secure Token isn't required anymore and now they're forcing domain validation. At least that's what I got out of this
You are receiving this email because your site is using the reCAPTCHA secure token. This feature has been deprecated and will be turned off on May 18th, 2016.
- If you have a small number of domains, please list them in the admin console domain box.
- If you cannot list every domain or have greater than 50 domains, you can find “Advanced Settings” in the admin console and untick the “Domain Name Validation” checkbox.
After that, your current key will continue working for all of the domains you are hosting but to maintain security, you are required to check the hostname of the solution on your side.
Please let us know if you have any questions and we are glad to help.