What version of Outlook are you using?
The following Technet/Knowledge base article
may help:
OL2000: Cannot Open iCalendar Format (*.ics) File
http://support.microsoft.com/?kbid=823199
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Nikki Peterson [MVP - Outlook]
I have had exactly the same problem since I upgraded from 2000 to 2003. I
have done what you recommended and it does not work.
Worse, I can invite people to meetings and get their answer in the
appropriate format (iclaendar) . It is only when the invite originates from
somebody that I get it in a format for which I cannot accept orthat does not
get reported in my calendar.
Thanks for helping