This is a quote from (of all places) the National Plant Diagnostic Network's handbook on email security.
Information leaks are also a problem with email. Emails are often forwarded beyond the original list of recipients — have you ever received an email from a friend with a confidentiality or copyright notice at the bottom? This is an attempt by your friend’s employer to make sure that confidential information does not get forwarded on to someone who is not supposed to see it. Unfortunately, it is so easy to send email that information spreads beyond what you intend, especially if the information is confidential, classified or newsworthy. Sending out confidential information of any kind in an email can be especially dangerous and costly.
As a rule, people do send out private and confidential information via email all the time. It's one of the risks we take when engaging in public discourse on a not-too-public mailing list, and it's one of the challenges of balancing convenience with safety. If you had to arrange a dead drop in person to convey information to everyone in your extended circle of friends it would get very tedious very fast.
The problem is particularly challenging when the information is not just confidential but also newsworthy. News tends to leak out (cf. Snowden) in ways that aren't always comfortable to those who have had their confidences betrayed. Even people who should know better - those public officials whose correspondence on official matters is subject to FOIA - protest vigorously when their email is made a matter of public record. Yet we continue on, using open wires and easy-to-forward messages to do our daily business.