When composing a new message, select the Options tab and press the "Show BCC" and "Show From" buttons to enable them.

Options tab of the Message window showing the Bcc and From commands in the Classic Ribbon.

When you are using the Simplified Ribbon, you’ll need to press the ellipsis (…) menu on the right to see the commands.

Options tab of the Message window showing the Bcc and From commands in the Simplified Ribbon.

Enable Bcc and From field when using the Reading Pane

Reading Pane buttonWhen you reply or forward an email from the Reading Pane, the Show Fields group can be found on the Message tab.

When you are using the Simplified Ribbon, you’ll need to press the ellipsis (…) menu on the right to see the commands.