Have I been pwned? Check if your email has been compromised in a data breach

"How to collect e-mail addresses" - Exercise 1

This is an impressive service (the speed, especially)...collation of different data sources into one easily accessible form is a hugely useful and underrated service. That said...there's no such thing as better security without an equal tradeoff. Here, it's now much easier (especially with the site response speed) for a third party to look up email addresses, see who they've patronized, and aggregate them into a database of less noble intent.

To check if a given email address was an Adobe/Gawker/whatever customer, you would've not only had to query every separate form but you would also not be guaranteed to get a definitive response (because some services will be ambiguous to whether you got a password wrong or whether the account exists at all). With the OP's service, with positive hits, you not only get confirmation of patronage, but knowledge that they are vulnerable, even if in a small, outdated way.

It's likely something Troy has anticipated but didn't want to outright say...In the end, knowledge is better than ignorance, and the correct response is for more rapid response to hacked victims and better security awareness. But I also wonder if there's a way to provide the OP's service with more (beneficial) obfuscation?

Have you been pawned? Yes, now that you just gave us your email address to sell to Russian hackers.

If you use LastPass, then their security check will run these checks automatically against all leaks and email addresses:

http://blog.lastpass.com/2012/01/new-years-resolutions-with-...

But it's not public information either, unless you choose to make it public.