Your users already are burdened with working for a company that has managers that want to know if someone read an email. This is CYA at its best!
Make it as hard as possible, so someone in authority can see how stupid this is.
Put a link to a website where the user has to login every time, search for the message based on a GUID contained in the email body that is one of those images with the crazy letters no one can read, to verify after they have left a blood sample on the finger print reader of their IBM Think Pad that has been disabled by the IT Department because they did a complete non-factory install of Windows XP Pro which does not contain all of the hardware drivers, and then send a confirmation email to each of their managers requiring a confirmation that they received the confirmation.
