***Disclaimer*** - This is definitely a rant, but I think we should all take a moment to reflect on our emailing habits.
Work email is a fun topic because everyone abuses it, myself included. It is far and away one of the biggest disturbances throughout the day. I would be willing to bet you're either nodding incessantly, or screaming "What the heck are you talking about!?" at the monitor as you read this. Either way, I would be glad to elaborate.
Problem: The Cc Game
You might Cc someone because you think the person in the To field will respond better if they see that someone else has been invited to the conversation. That's wrong, don't do it. The people who get included on these emails can't get to other emails that are actually important, or may skip over the wrong emails. If you feel like that is the only way to get an answer, then email is probably the wrong medium of communication.
CYA is a culprit motive of the Cc game, but we shouldn't have to feel like we need to cover our footprints with everything we do. If you feel that way, it's probably a good idea to discuss it with someone.
Another reason to Cc someone (or better yet, a mailing list) is a shift of responsibility. For all intents and purposes, it's like raising a flag to identify a problem, and then offloading the burden to all included parties. Again, don't. If you need help, you should have a prioritized list of people to talk to about your problem. If you don't know who to talk to, don't use the spaghetti test (throw it at the wall and see what sticks). Ask someone else who might know who you should ask. Sometimes, there's even a process to dictate that for you.
A side effect of playing the Cc game is that people hit the Reply To All button. If you're going to hit Reply To All, take a second to skim the recipients, and knock off the unnecessary ones.
Funny side note: I saw an email today that had individually listed a few of my team members and myself in the To line, and then our emailing list in the Cc field. Redundant!
To be clear: email is not unnecessary, but its misuse is. Good communication has a strong correlation with success but it can be a double edged sword. Over communication is distracting, and has a direct negative effect on productivity. One general piece of advice I found while googling this topic was to only include recipients if you would actually call them on the phone about the issue at hand. If you wouldn't include all of my team members in a meeting about your priority, then by Modus Ponens it is also unnecessary to include them in the Cc.
I'm not alone with this opinion, others feel this way too. Many other people. Lots of them. This is all subjective, and is endlessly debatable. I offer no concrete advice on when specific behavior is appropriate, but instead encourage you to think about who is on the receiving end before you press "Send," because I will.