Is it possible to scrape sig-up emails?

Hello Ladies and Gents!

Someone asked me today:

Is there a program/way one can get the emails people used to sign up on a website?

The question came during a discussion regarding how good can we, as business owners/site admins, protect our clients/users data.

I didn't know the answer and Google doesn't really try it's best to show relevant results regarding ways to steal information...

Any input is greatly appreciated! Thanks!