Have you ever replied to an email message only to realize too late that you just sent your reply to the entire department? Or worse, to the entire company? Reply to All should be used only when you are sure that every recipient on the list really wants and needs to read your reply.
Unfortunately, accidentally hitting Reply to All is an easy mistake to make.
If you are the sender of the original message, you can make life easier and safer for your readers if you use the bcc: field instead of the To: field in the email header. Bcc: stands for "blind carbon copy. Every user will receive the message but the recipient will see only his or her own name in the bcc: field. If a user accidentally hits the Reply to All button, the reply message will only be sent to the original sender.
As a matter of ettiquette, you should disclose the distribution list to your readers in the body of the message. This avoids any appearance of attempting to hide the distribution. A common convention is to use small italicized text in the first line with the text "sent bcc: to MidwestDivision".