BrowserSpy.dk shows you just how much information can be retrieved from your browser just by visiting a page.
Available tests are listed below.
Email Address Verification Information
Find out if your email address can be verified? Some mailservers are setup so that the validity of a email address can be verified.
Sorry, but I haven't had the time to port this check to the new platform yet. You might want to consider donating so that I can take the time.