Depends if you are using Word or Outlook as the email editor.
All the answers given so far are correct for OL as the editor. If you use
Word, it is different.
Straight out of Outlook's help menu: "You can display the Bcc box in all new
messages that you compose. If Word is your e-mail editor, in a new message,
click the arrow to the right of the Options button, and then click Bcc. If
you are using the Outlook e-mail editor, in a new message, on the View menu,
click Bcc Field."
Good luck.