Outlook itself keeps track of the opened files. There is no attribute for the message itself at the server level you can set from an external system like MII.
The Read_Mail or Send_Mail functions in MII mimic a basic POP3 client (if you have the option to leave the mail on the server). IMAP connectivity works directly with the server content and its the preferred way of keeping things interactive. If the server does not detect when a message is read by any client logging in, accessing the information from it and update the messages statuses automatically, there is little to do in MII to enforce that thing to happen.
Regards,
Paul.