Ok, I know we can open the pst and extract a message, but the question remains, can you take that newly composed message and make it look like the original, all while keeping the original time stamp for the message, and write the information back into the original pst file without having a new modified date.

Unfortunately that is not possible to do

Is there a way to prove it is not possible? I have a client in the midst of a legal issue, and this would be the concrete proof that is needed.

Thanks

It's not possible for a normal Outlook user to alter a message, but in theory, someone who knows enough programming and .pst file format should be able to do it, it's just a file after all.

It also depends on the encryption method. Check out this link, scroll down to Encryption Settings. According to this article, it seems that even the 'Best Encryption' only prevents editing by a text or hex editor.