Currently, we can only access the `bcc` property of a Mailbox Item if we are in the Message Compose context. This usually makes sense, since when we're in Message Read mode, we're probably reading emails sent to the user, so the user naturally shouldn't see BCC addresses. However, in the Sent folder, we should be able to access the `bcc` properties of the messages there, as they themselves should have set them.
In our use case, our Addin would surface additional useful information to the user contingent on all of the recipients they've sent a message to (`to`, `cc`, _and_ `bcc`). I don't believe there is any known workaround for developers to access the `bcc` property of a Message while in the Sent folder.