According to a recent update to Google Apps, the Gmail platform will now be able to scan documents inside of emails using optical character recognition and match it against an admin-approved blacklist. This means that before an email is sent out, it will be checked against certain words, phrases, or numbers.
If for example a number like a social security number or a credit card number was detected, it will stop the email from being sent out. This is help prevent accidental leaks, or in some cases, can be used to help trace evidence back to employees who might be leaking information to the competition or to themselves on the sly.
However we should note that this feature will only be available to paid Google Apps Unlimited customers and is more targeted at businesses rather than individual Gmail accounts. However if you run a business and would like to check it out, hit up Google’s website for the details.