For those who don't know what this is, BCC (Blind Carbon Copy) is a protocol on every e-mail message service (Gmail; Yahoo; Hotmail; Etc.) that allows the sender, when he is sending an email to several contacts at the same time, to hide the contacts that he chooses. The contacts the sender writes on the "To:" and the "CC:" will be visible to everyone, but the ones on the "BCC:" will be hidden from everyone.
Now I'll show you how to add BCC to your emails on Gmail; Yahoo and Hotmail:
I don't have any print for hotmail, but it's on the right-up conner of the e-mail message.