D. J. Bernstein
Internet mail
Internet mail message header format

Recipients: To, Cc, Bcc

To

The value of a To field is tokenizable. It is an address list. For example: 
     To: The Boss <God@heaven.af.mil>,
         <angels@heaven.af.mil>,
         Wilt . (the  Stilt) Chamberlain@NBA.US
Note that Pine misinterprets the last address; writers should encode the address as 
         Wilt.Chamberlain@NBA.US
instead.

According to 822, the To field ``contains the identity of the primary recipients of the message.''

822 requires that every message contain at least one To, Cc, or Bcc field. This poses problems in practice for messages without any visible recipients. Users occasionally leave out all recipient fields; unfortunately, if a message without To/Cc/Bcc passes through sendmail, sendmail will put the recipient list into a new Apparently-To field. One solution, used by qmail, is 

     Cc: recipient list not shown: ;

Multiple To fields

Some MUAs will on occasion generate a message with two or more To fields (typically because sendmail V5 cannot handle messages with more than 8192 bytes in a To field). I recommend that readers accept multiple To fields and interpret them the same way as a single To field with all the addresses.

However, writers cannot rely on this interpretation. The semantics of multiple To fields were not specified in 822. When Eudora looks for a To field, it takes only the first one.

Cc

The value of a Cc field is tokenizable. It is an address list.

Cc stands for ``carbon copy.'' According to 822, the Cc field ``contains the identity of the secondary (informational) recipients of the message.'' According to 822bis, the Cc field ``contains the addresses of others who should receive the message, though the content of the message may not be directed at them.''

Bcc

The value of a Bcc field is tokenizable. It is either an address list or a series of zero or more commas.

Bcc stands for ``blind carbon copy.'' In practice, a user types Bcc to tell his MUA to send the message to extra recipients not shown in To/Cc. The MUA removes the Bcc field before sending the message, so the To/Cc recipients have no indication that there were any Bcc recipients.

822 suggests that MUAs can leave Bcc in the copies sent to the Bcc recipients. However, this usually fails in practice, since sendmail strips Bcc out of any message it sees. Users who want to reveal the Bcc recipients to each other should send two messages: an original copy to the To/Cc recipients, and then a forwarded copy to the extra recipients.