Please keep replies to the list so that others can participate.
I sugest the notify, because the admin can know if a user open a email, copy to another folder, delete the email....etc.
but using the MTA it can be a solution too.
At least in Germany, doing this is quite risky as this gives lots of information about the person. German law limits the use of profiles like these quite strictly; in general an employer has no business to log all these actions generally. This does not mean it can be helpful for diagnosing problems, but I would not recommend to do so as a general procedure. On the other hands, it is a de-facto requirement today that you can trace every communiction that has entered the company. The only place where you can do this without potentiall missing some messages is the MTA. This is why all the archival tools want to couple to the MTA...
JC