In Outlook you can show, hide, and view the Bcc (blind carbon copy) box in an email message. Only the sender of a message sees the names of people (recipients) in the Bcc box. But everyone can see recipients in the To and Cc boxes. Anyone added to the Bcc box receives a copy of the message.
Turn on Bcc
After you turn on the Bcc box, it appears every time you create a message until it's turned off.
Create a new email message, choose Options tab, and in the Show Fields group, choose Bcc.
In the Bcc box, add recipients, compose the message, and choose Send when done.
Turn off Bcc
You can turn off the Bcc box for future messages at any time.
Create a new email message, choose Options >Bcc.
Note: The background color distinguishes between the Bcc button being turned on or turned off. A dark background means the button is turned on, and a light background means it's turned off.
View Bcc recipient
If you're the recipient of a message and want to see if the sender added Bcc recipients, you can't. Only the sender of a message can see the names of Bcc recipients by opening a message in the Sent Items folder where all sent messages are stored by default.
In the Sent Items folder, open the message you sent.
In the Reading Pane, view the header section of the message.
Tips: A + symbol indicates there are more recipients. In the screenshot, for example, +11 signals there are 11 more recipients listed in the Bcc box.
Choose to expand (open) your the message header section. And choose to collapse (close) the header section.