Hi everyone,
This time I want to talk about the Recaptcha anti-spam. Yes, the one created by Google.
It is great when it works, but it may become a nightmare if you have restrictions in your environment (especially if you are working on SharePoint) 
Problem
If you get the message “The reCAPTCHA server is unavailable”, it means that the attempt to connect to Google to validate the Response (the text you have inputted) failed. If you are referencing the binary Recaptcha.dll in your solution I assure you it is going to be very difficult to identify the source of the error.
Solution
Lots of factors may lead to errors. The most common thing is that you may have a proxy in your environment, so you need to set the proxy as a reference in the Recaptcha control.
To troubleshoot that, start by checking the EventViewer, as there will be a message under the “Application” source.
Other unknown factors may possibly affect that, so I recommend you to get the source code from Google:
http://code.google.com/p/recaptcha/source/checkout
Install TortoiseSVN and get the source code, then add it to your solution. By doing this you can debug and check what is going on, otherwise it is going to be very difficult to understand this generic message from the EventViewer.
I hope it helps.
Cheers,
Marcel Medina
Click here to read the same content in Portuguese.