Bcc stands for Blind Carbon Copy. You typically BCC someone on an email when you don't want any of the other recipients to know someone else is being copied on the email. Unfortunately there is a bug in System.Net.Mail when you BCC someone. If you look at the raw headers of the sent mail message you will see X-Receiver header for everyone that was sent the message. This includes the To, CC, and BCC addresses. Thus anyone who was BCC'd on the message is recorded in headers.