Bcc is a blind copy and is not displayed in the headers of the received message. This is why it is so difficult to test and debug it. Keeping SMTP rules in mind, to check Bcc addresses, you should compare headers of the received messages (DATA) and the recipients (RCPT TO).

Does email header Show BCC?

Conversely, Blind Carbon Copy or "BCC" indicates that the recipient also received a "copy" of an email that was sent to another recipient, however, the recipients do not know to who the email was copied. The email's header will include only "To". The BCC recipient's email address is not visible to the other recipients.

How can you tell if someone was BCC on an email?

When you receive an email, you can check if you are in the “To” or “Cc” field. If your email address is not appearing in either the “To” or “Cc” field, that means you are a Bcc recipient.