Blind Carbon Copy - Visibility

Visibility

In most implementations, the recipient of an email can see any email address specified by the Sender in the To: or Cc: fields. If on the other hand the Sender has specified addresses in the Bcc: field, the recipient in this case cannot see these Bcc addresses.

The internet standard for e-mail messages is RFC 2822 and the Bcc: header is discussed in section 3.6.3. It is unclear whether Bcc: is designed to ensure the Bcc: addresses are hidden from each other. On the one hand, it says:

The "BCC:" field (where the "BCC" means "Blind Carbon Copy") contains addresses of recipients of the message whose addresses are not to be revealed to other recipients of the message.

It also states:

There are three ways in which the "BCC:" field is used.

1. In the first case, when a message containing a "BCC:" field is prepared to be sent, the "BCC:" line is removed even though all of the recipients (including those specified in the "BCC:" field) are sent a copy of the message.
2. In the second case, recipients specified in the "To:" and "CC:" lines each are sent a copy of the message with the "BCC:" line removed as above, but the recipients on the "BCC:" line get a separate copy of the message containing a "BCC:" line. (When there are multiple recipient addresses in the "BCC:" field, some implementations actually send a separate copy of the message to each recipient with a "BCC:" containing only the address of that particular recipient.)
3. Finally, since a "BCC:" field may contain no addresses, a "BCC:" field can be sent without any addresses indicating to the recipients that blind copies were sent to someone.

Which method to use with Bcc: fields is implementation dependent and may depend on both one's mail user agent (e.g. Outlook, Thunderbird) and mail submission agent (usually provided by one's ISP).

Since the hiding of the Bcc: addresses from other Bcc: addresses is not required by RFC 2822, one cannot assume the Bcc: addresses will be hidden from other Bcc: addresses