A blind carbon copy is a copy of the mail message that is sent to one or more recipients without the knowledge of the primary recipients. Primary recipients are listed in the To: and Cc: lines. When there are multiple blind carbon copy recipients, knowledge of each other is also hidden.

When run with a -t command-line switch (to gather recipients from the headers), the sendmail program achieves this end by saving a list of all the blind carbon copy recipients, deleting the Bcc: header line, and then delivering to each blind carbon copy recipient. (See the Apparently-To: header.)

The Bcc: header should never be declared in the configuration file.

The field for the Bcc: header must contain one or more properly formed addresses. Where there is more than one, each should be separated from the others by commas.