There are a few reasons to use "Bcc," most of them bad. Misuse of the email tool can make personal correspondence read like a spam, make party invites seem sneaky and turn co-workers into double agents.
Bcc stands for "blind carbon copy," and is a way of sending emails to multiple people without them knowing who else is getting the email. Any email addresses in the Bcc field will be invisible to everyone else on the email. In other words, it's like cc, but for spies.
The only good time to use Bcc when sending an email is if you are sending something impersonal (change of address, selling a dresser) to a lot of people who don't necessarily know each other. No one is pretending that this email is anything but informational, so it's fine to hide the other people on it. Also, the number of people included on an email like this might number in the hundreds, and no one wants to scroll through that many names. As a rule of thumb, if the number of recipients exceeds 30, then you should Bcc.
The worst time to use Bcc is at work. It's shady to lead someone to believe they are the only recipient of an email when they are not. The cc ("carbon copy") connotes the same thing as the "Bcc" -- the cc'd person is on the email but is not expected to respond -- but it does so openly. If you need to copy your boss on an email, copy your boss, but don't pretend they aren't copied.
In personal correspondence, a bad time to use Bcc is when sending an invitation to a party. Are your friends spies? Can their identities not be revealed to your other friends? Are your friends so famous that your other friends will stalk them if they know their email addresses? Bcc'ing in this instance can make you look insecure and offend your friends. Insecure because it seems like you want to keep who you are inviting to the party a secret, because A) it'a a big party with a good number of losers invited, or B) it's a small party with few people invited, but some of them are definitely losers. And it's insulting because it makes it seem like you don't think the people you’re inviting are sane enough not to immediately start spamming everyone else on the email. Don't try to pull the wool over your friends' eyes! It's nice to see who else is invited to a party because then you know how many losers will be there!
Bcc'ing on an invite email makes intimate events among friends seem like corporate events. However, if the event in question really is more like a conference than an intimate gathering (you rented a bar, hall, ship, etc.) Bcc'ing is fine since once you've got 200 people on an email -- even listing their names isn't going to make it seem personal. Better not to annoy invitees with a huge list of email addresses.
Another time not to use Bcc in personal correspondence is when sending a group email life announcement like "Yippee! Got the job!"
There is a difference between a mass email which is generally informational and spans a wide range or contacts (friends, coworkers, family) and a group email, which usually has more personal content and is being sent to people who, while they may not necessarily know each other, exist within the same social realm.
When you Bcc a group on emails containing life announcements, it throws people off because the first reaction to an email from a friend is warm, fuzzy inclusion, a feeling that is killed as soon as they open the message to find it's to "undisclosed recipients." It's better to make this an email where all names are shown, then even though the recipient is not the sole target of the message at least they know they are part of a club and not a mailing list.
If you're really worried about preserving people's email privacy, send these messages in groups to people who you are sure know each other, then send individually to the secret people your friends don't know you're friends with.
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