However, if you do intend to send a mass missive, consider whether you should list recipients in the CC or BCC fields. If it’s a routine work email that is going out to a few colleagues, CC keeps everyone in the loop as to who is receiving the message. It’s clear and straightforward.
But if you are sending an email to a variety of people—say, assorted friends, or people who don’t know each other—it’s better to BCC. A journalist friend of mine once received an email from a PR that had every major media person’s email visible, which is something that is a major no-no. Most people prefer to keep their emails private, for good reason.
If you are responding to a mass email, only click “reply all” if your message is pertinent to all parties. If you simply want to send a private note to the original sender, hit reply, and only he or she will see it. It drives me crazy when my email in-box is flooded with “reply all” messages that don’t involve me at all. A mass email is meant to save time for the sender, not create a bigger nuisance for the recipients!