It has become apparent to me in recent days that people just don't understand how to properly reply to emails. I won't even get into all the people who don't reply, right now I am focused on the huge difference between reply and reply to all.
Someone sends an email accidentally to a list that includes the entire company. Various people across the world, hit reply to all to explain that they believe that they were sent the email erroneously.
Suggestion: If for some reason you feel certain that the thousands (not exaggerating here) of people who received this email have not already communicated the error to the sender, hit reply, and let the sender know. Just the sender. All of the other people who didn't need this email don't need one from you.
Someone sends an email to a large team, introducing new members. Two members have worked together before on another project so one hits reply to all to say hello and catch up and mentions information will follow a little later thereby leading to confused responses from some of the other people who don't understand what is being asked.
Suggestion: Hit reply, or hit reply to all, but strip off some of the other people who do not need to know that you are buddies or that you are sending someone else information later.
Someone sends an email to a listserve asking for assistance. Someone hits reply to all, offering possible assistance and personal health and financial information.
Suggestion: This one is admittedly gray. Perhaps the replier really did intend to share all of that info with the entire listserve. More likely they mistook it for a smaller listserve or didn't realize they hit reply to all. They may have felt that others needed to know assistance had been offered, and maybe one email to the list with a follow up to the person would have been more appropriate.
Someone sends an email requesting everyone email them with available times. Everyone hits reply to all.
Suggestion: Again there is some gray here. I find it usually works best to narrow the time - to a specific day, or a few suggestions. Often it makes sense to have one person collect times and reply back to the whole group with the best choice or top two for review. But really, I don't care if Wednesday is better for you or Friday for you, mostly I care when we are meeting.
As with other technology, reply to all is meant to help you. Certainly I tend to err on the side of using reply. But you can also hit reply to all and strip out people. Or send a short response to the whole group and follow up on specifics with the people who really need the info. We're all really busy. We all get too many emails. Help us out.