What you didn't tell is if your company uses MS Exchange Server. That's likely. Assuming the system manager does something like a backup the original mail can be found on the backup, even if you manage to produce a fake copy. So any change compared with the original can easily be detected as a fraud.

Of course, if you forward a mail from your Sent Items box, you can edit the contents. That's easy. So everybody knows not to trust forwarded emails if it's important. Say, you simply add your boss' email address to the list of people having received a cc. All he has to do: visit one of the original receivers and print the mail there. His name won't be in the cc in that original copy. Guess what he wll conclude. Guess what he will do.

Even more tricky: add a Bcc line with your boss' email adress in the forwarded mail. That line won't show up in the copies from the original receivers, but that's not to be expected. However, if you use a BCC it won't show if you forward the copy in your Sent Items. So if it appears on the forwarded copy, it's fraud by definition. Guess the consequences.

So you need a more involved approach and that can't be done inside Outlook. The only solution then: export, falsify using some tool (not Outlook) so it appears like it always had the BCC and import again.
But if he then compares it with the original on the backup of the Exchange server, you hang.

And, alas (or luckily for me) forum policies forbid discussing illegal activities as falsifying emails clearly would be. The forum policies (link at left) are quite clear about that.
So - moreover - I'm locking this thread.