You would think that everyone knows how to use email by now, but it has a lot of basic etiquette rules that people still don't follow. Productivity weblog GTD Times points out a simple one: knowing the difference between "To" and "CC".
One of the most important strategies for dealing with inbox overload is making your messages easy to act on. In fact, this is one of our top email values. GTD explains how these two fields can help you achieve this goal:
Be discerning about your use of To: vs. Cc:. Why? Ever receive an email where it's unclear who has the action because everyone is in the "To:" field? We designate the To: field for who has the action (could be multiple people). Cc: is simply for their information -- with no expectation that they will take action on the email, other than receive it. Personally, I find I am much more conscious about what I am asking for, and from whom, when I clearly delineate between who has action and who just needs to receive the information. And, I appreciate when that distinction is made for me in return.
If you're good about your email, you've already been doing this for years. But do your coworkers? If not, a simple heads-up on the etiquette surrounding this feature could really help you out. While you're at it, you should let them know whether you expect them to reply all too.
Email Best Practices for Teams [GTD Times]