Blind carbon copy (abbreviated Bcc:) allows the sender of a message to conceal the person entered in the Bcc: field from the other recipients. This concept originally applied to paper correspondence and now also applies to email.
[Stout, Chris. "DEAR NERD: Blind carbons hide addresses." Charleston Gazette (West Virginia, USA). 1998-01-18. page P5B. NewsBank record number 100F35638A890441.]
In some circumstances, the typist creating a paper correspondence must ensure that multiple recipients of such a document do not see the names of other recipients. To achieve this, the typist can:
Add the names in a second step to each copy, without carbon paper;
Set the ribbon not to strike the paper, which leaves names off the top copy (but may leave letter impressions on the paper).
With email, recipients of a message are specified using addresses in any of these three fields:
To: Primary recipients
Cc: Carbon copy to secondary recipients—other interested parties
Bcc: Blind carbon copy to tertiary recipients who receive the message. The primary and secondary recipients cannot see the tertiary recipients. Depending on email software, the tertiary recipients may only see their own email address in Bcc, or they may see the email addresses of all primary and secondary recipients.
It is common practice to use the Bcc: field when addressing a very long list of recipients, or a list of recipients that should not (necessarily) know each other, e.g. in mailing lists.
[Husted, Bill. "Bad e-mail habits can be bothersome, embarrassing" Atlanta Journal-Constitution, The (Georgia, USA). 2009-08-30. page E15. NewsBank record number 103419444.]
There are a number of reasons for using this feature:
Bcc is often used to prevent an accidental "Reply All" from sending a reply intended for only the originator of the message to the entire recipient list.
[ NewsBank record number 200908250100KNRIDDERFLMIAMIH_poked-08-25-09.]
To send a copy of one's correspondence to a third party (for example, a colleague) when one does not want to let the recipient know that this is being done (or when one does not want the recipient to know the third party's e-mail address, assuming the other recipient is in the To: or Cc: fields).
To send a message to multiple parties with none of them knowing the other recipients. This can be accomplished by addressing a message to oneself and filling in the actual intended recipients in the Bcc: field. However, this does not ensure that the Bcc: addresses will be hidden from other Bcc: addresses in all implementations.
To prevent the spread of , spam, and malware by avoiding the accumulation of block-list e-mail addresses available to all Bcc: recipients, which often occurs in the form of chain letters.
In some cases, use of Blind Carbon Copy may be viewed as mildly unethical. The original addressee of the mail (To: address) is left under the impression that communication is proceeding between the known parties, and is knowingly kept unaware of others participating in the primary communication.
A related risk is that by (unintentional) use of 'reply to all' functionality by someone on Bcc, the original addressee is (inadvertently) made aware of this participation. For this reason, it is in some cases better to separately forward the original e-mail.
Depending on the particular email software used, the recipient may or may not know that the message has been sent via Bcc. In some cases, ‘undisclosed recipients’ placed in the To: line (by the software) shows that Bcc has been used. In other cases, the message appears identical to one sent to a single addressee. The recipient does not necessarily see the email address (and real name, if any) originally placed in the To: line.
When it is useful for the recipients to know who else has received a Bcc message,
their real names, but not their email addresses, can be listed in the body of the message, or
a meaningful substitute for the names can be placed in the body of the message, e.g. ‘To’, or ‘To’.
Carbon vs. courtesy
The interpretation of "Bcc:" as "blind courtesy copy" is a backronym
and not the original meaning; the historic RFC 733 has an explicit "blind carbon" annotation in its definition of the Bcc: header field syntax. "Cc:" and "Bcc:" mean "carbon copy" and "blind carbon copy" respectively.
Sending courtesy copies of mailing list replies also directly to the author(s) of answered message(s) is a common practice on some lists, and matches a new interpretation of "Cc:" as abbreviation for "courtesy copy".