This cutsie graphic going around contains an important lesson. When you send an email to a group using TO or CC, everyone in the group can see everyone else’s email addresses. You might think, So what? We’re all friends. Yes, but what about when your friend forwards the message on to a few dozen or a few hundred of his friends? Do you really want all of them to have your email address? If any one of those friends-of-friends is infected by a spambot — BINGO, you all start getting spam.
In case you think this is an unlikely problem, it happened to me this Christmas. I received a Merry Christmas email blast from someone I’ve never met, who lives thousands of miles away — a relative of a co-worker. The happy email was addressed to hundreds of people, and all of their email addresses were visible. Shortly after that, I started getting a LOT more spam.
Another reason to use BCC when emailing a group is to prevent an Unsubscribe Storm. This starts when a large mailing gets some smart-ass, controversial, or off-topic responses. The responders use reply-all, so everyone can see their brilliance. This annoys people who are now getting a lot of messages they don’t want, so some of them also reply to all, saying “Take me off this list”. At that point, nearly everyone wants off the list, and the volume of messages saying so becomes really alarming.
It sounds funny, but it can get really out of hand. I saw a case where a message like this had a large attachment. The resulting volume rendered heavy-duty corporate mail servers unusable for the rest of the day, and the mess took a while to clean up.
If BCC had been used, then reply-all would only reply to the sender, and no harm would be done.