This is a bit of a shot in the dark, but has anything changed recently in the distro that would now permit external email containing a calendar appointment to display correctly in the email client (Outlook, in this case), showing the Accept, Decline, etc. buttons?
One of my clients' staff members, an engineer, is adamant that up till recently, such externally-sourced appointments displayed as emails without the ability to accept, etc. The client has not updated software on the desktop, but I have been applying the various server updates over time.
Thanks for any ideas....
Des