個人用メール、予定表、連絡先管理に従来の Outlook on Windows を使用する
下記の文書を参照してください。
Outlook: BCC field is not displayed on some items in the Sent Items folder
Symptoms
When you look at an e-mail message in your Sent Items folder you may not see the Bcc field on the item even if you included e-mail addresses in the Bcc field when you originally sent the message.
Cause