Understanding BCC (Blind Carbon Copy)
When an email is sent with BCC recipients, the BCC email addresses are not listed in the header. The “To” recipients are listed in the “To” header field, and the “CC” recipients are listed in the “CC” header field, but the BCC recipients are intentionally left out. This is what makes it a “blind” carbon copy — the recipients are unable to see the BCC recipients. (It is by design.) Otherwise the BCC recipients would be equivalent to “CC” recipients. To understand how BCC recipients receive an email, I recommend reading this blog post: SMTP Protocol in a Nutshell. The BCC recipients are passed to the SMTP server in “RCPT TO” commands.