The Outlook add-on Bccthis (pronounced as two words, "bcc this") works off the assumption that e-mail should be like interpersonal conversations down to the part where some of you talk behind the others' backs. As a result, the Bccthis window that appears below Outlook's main message window gives you a space to pen a secondary, private message to one or more recipients, making the new message literally subtext. All Bccthis recipients get two e-mails for the price of your one: the original e-mail that's visible to all, and the "blind," secondary e-mail that only some people see.
In reality, Bccthis is only as snarky or as sinister as you want it to be. In a professional context, it could also be a time-saving tool wherein one can supply extra direction to a subset of the contact list without burdening everyone.
Bccthis is expandable and collapsible, so you can hide it when it's not in use. A separate formatting bar on the Bccthis pane lets you compose with the usual flair. We didn't always receive messages in the correct order, and we'd like to see a few more customization options for outgoing messages, like more say over visual treatments, and if you want outgoing messages consolidated into one as well as your incoming Bccthis messages. Though it isn't going to be useful for everyone, those who frequently submit batches of messages to a subset of a group could save some time and energy with the Bccthis add-on.