know to whom you are replying

notice who will receive messages you reply to.  if you are replying to a birth announcement sent to 200 people, perhaps all 200 don't need know that you congratulate the parents.  if you are making dinner plans with five people, be sure all five know that the reservations have been moved to 8:30.  keep an eye out for mailing lists, maybe your message is not intended for everyone on the list.

most e-mail clients have "reply" and "reply all" options.  choose wisely.  feel free to add and remove recipients as you see fit.  you have control over who receives your messages.