There is a 3rd party service that some of my old law firm clients were considering that effectively does provide tracking of email receipts. One of the more popular ones is RPost. It is not fool-proof as they mostly guarantee only that the message makes it to the recipient's server, not to the actual recipient. I believe they also have a mode that embeds a tracking image in the email but that also depends on the client permitting the display of the image. Either way is slightly better than the built-in Outlook options.

The interesting thing about RPost is that they seem to have received some guidance from lawyers such that they consider their service "proof of delivery" in the legal sense.