Show the “Bcc” or “From” box
Anyone added to the Bcc box receives a copy of the message. People in the To and Cc boxes won’t see the names for anyone listed in the Bcc box.
When you first use Outlook, the Bcc and From box doesn’t appear. After you turn on the Bcc or From box, it appears every time you create a message.
If you want to turn off the Bcc or From box for all future messages, when composing a message, click Options > Bcc.