Secure Outlook Email
Normally when email received popup will be shown on windows UI for new email from sender and with subject.
Sometime you are sending some confidential email and your intention is other not to see and there should not be any popup on UI and till the time email not opened, all content remain marked as xxxxxxxx. This is needed as sometime Receiver may be projecting her/his screen to others and all will come to know for email if popup is displayed or receiver going through emails in inbox.
May be some kind of option in email compose can help to mark email secure.