What is BCC and how is it implemented?

I occasionally receive some special emails in my email box. Checking the emails I could not find my email  address listed in the “To:” header of the emails. Why can I receive those emails that are not destined to my email address? In fact, that is where Bcc(Blind Carbon Copy) plays its role.  If you fill Tom’s email address in the Send-To box and Joe’s email address in the Bcc box when composing an email . Both Tom and Joe can receive the email. However, Tom can not see Joe’s email address(in the recipient list) and Joe can not even see his own email address in the  recipient list(but Joe can see Tom’s email address in the recipient list). How can this be done?

When sending an email, an email client first sends the envelope information using SMTP command “MAIL FROM” and “RCPT TO”. Both Tom and Joe’s email addresses are sent by the “RCPT TO” command without a difference. The difference appears when the email client builds the message headers. Tom’s email address will be built into the “To:” header while Joe’s email address will not. We know that the envelope “RCPT TO” will be used by SMTP servers to deliver the email so the email can be sent to both Tom and Joe without problem. But when Joe and Tom use an email client to check the email, the email client only displays the message which not includes Joe’s email address.

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