Sometimes even the most innocent little thing has a security risk attached to it. When you are sent a good joke or, heaven forbid, a cool animated novelty email, it’s easy to imagine that many of your friends would like to see it, too. While some are legitimate, many of the novelty emails are hiding a malicious program that will infect machines when opened. Let’s assume that you, or your organization have had it drilled into your head not to open email attachments from people you don’t know or trust. That’s great. Now, take a look at the recipient list on those emails you have received and forwarded on to all your friends. Does it show the names and email addresses of others who received it? That’s a BAD thing.
One reason it’s bad is that it is revealing the names and email addresses of a lot of people to other people whose only connection to each other is probably the sender. This is not always welcomed by everyone. Not everyone wants to be widely reachable by email.

The other reason it’s bad is that eventually that message with all those email addresses will find its way into a SPAMMER. How? Any one of the recipients who doesn’t have up to date antivirus software running can have a virus, worm or Trojan Horse program they don’t know about running on their computer. These nasty little things can read your email and address books, just looking for addresses to send SPAM to.

So, when you, or someone in your organization has the urge to forward an amusing email, please use the Blind Carbon Copy feature in your email software. It’s sometimes shown as BCC: or Bcc: and it hides the email addresses of anyone else the message was sent to where the sender put their address in that field. It’s a good way to keep those addresses out of the hands of unintended recipients.

For more info see here.