Users of email often violate distribution rules regarding who exactly receives an email. Generally, this is botched in the CC and BCC fields.
Let me begin by explaining a situation that happened to me in the past. A listener of my "No Agenda" podcast sent an email to me and my co-host Adam Curry. The note described a situation that I thought would be interesting to discuss on the show in a way that might surprise my co-host.
I wrote directly back to the listener to tell him how much he helped me and how I planned to execute the surprise. He sent me back some meaningless continuation of the thread with some additional commentary. The entire thread was attached and he re-added Curry's email address to the CC, thus revealing the private correspondence between myself and the listener and spoiling any element of surprise.
In later correspondence, when I asked the listener why he re-added Curry, he said that he was on the original thread and thought he should continue to be on the thread. He seemed oblivious to the whole concept and thought he did nothing wrong.
Let me lay out the first rule right here: Do not add any names to the CC or BCC line after a thread is forked. It is a total violation of email etiquette. Don't do it.
Which brings us to a more interesting dilemma – the use of the BCC. It also has a lot of unwritten rules, the most important being that if you get an email as a BCC, then assume (unless you know otherwise) that you are supposed to keep quiet. BCC means nobody knows that you got the email chain in the first place.
Sometimes, there is a good reason for this. It's often done to alert you to a developing situation that someone else cannot let on that you know about it. Most of the time, it is so you can discuss it with members of your clique offline later. It is essentially a rude mechanism because it is secret.
For example, the CEO may get BCC'd on a crazy discussion between the head of engineering and one of the nutty engineers, so the CEO will not be totally surprised when a situation erupts later.
Which brings me to my second rule: If you are the recipient of a BCC, never respond to the thread. Ever. Don't jump in on the conversation. You are not part of the conversation and it will result in befuddlement in most instances. "How did that guy get in on this thread?"
It all ends up getting traced to the original or previous email and then it traces back to that email's sender. He or she, who was doing you a favour, is now considered a creep or agent provocateur for adding you secretly in the first place. If that person wanted you involved in the discussion, he or she would have put you on the main distribution list.
At the end of the day, you, as an email user, must assume that people will commonly violate both of the rules set forth here. So, as a third rule, assume the worst and avoid being chatty on any email that begins with multiple recipients. Always assume you will get yourself into trouble if a sensitive email that you wrote gets into the hands of a BCC recipient who you sent it to for whatever reason.
In other words, be more careful.
While you're here, you might also want to read our piece on email etiquette and the end-of-thread issue.