Add option to always BCC group members
Groups are often used for contact lists where it is important to maintain privacy. It would be good to have an option for each group to always BCC members, even if the group is in the To field. The current scenario requires users to remember to put the group in the BCC field every time. An "always bcc" option would prevent mistakes.
This suggestion isn't totally comprehensive, but it's essential that a future version of Outlook has better functionality around maintaining confidentiality. It's really too easy to make the mistake of using CC instead of BCC and we see well-publicized data breaches arising from this on a regular basis in the UK. Yes, it's user's fault if they use the wrong box, but perhaps the software can nudge people in the right direction somehow.