Right now, the email provider lets you know where and when you've signed in. If you're signed in on two or more different computers, that information is at the bottom of the page, and shows your IP address. That IP address is unique to every computer.
Starting now, GMail will take note of where you normally log in, and alert you if it's a new IP address. A different IP address could mean a hacker is in your account. Gmail will then offer to change your password immediately.
So far, GMail is the only email provider that offers this service.
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