Hacker and security researcher Samy Kamkar has a new tool out -- one that can find working email accounts for people at businesses, even if the address hasn't been published online.

The tool, called Peepmail, promises to deliver email addresses for everyone from Apple's Steve Jobs and Microsoft's Steve Ballmer to the random guy whose business card you lost. It takes advantage of the fact that many email servers will tell the sender whether the address is valid, even before the message is actually sent.

When...