There are many reports in the Google Web Search Help forums that Google Definitions feature is not working on some browsers for some people.

I cannot replicate it, but when you search for things like [what is toast], Google will provide this definition at the top:

Instead, users are getting errors that read:

I couldn’t replicate the issue myself, but here is an old screen shot of such an error:

Thing is, that email address is no long valid. Try emailing it and you will get a “Delivery Status Notification (Failure)” due to the “User unknown” or the user not being a real email account.

Kelly from Google said “Thanks for the info. We’re looking into it now.”

I am not sure if it is fixed or if they are still working on it at this point.

Forum discussion at Google Web Search Help.