This seems to pop up all the time by does not provide an answer:

I am using Oulook 2002 SP3. Pop account (not exchange).

When I receive calendar meetings over the INternet from other Outlook users
they show up as plain emails (i.e. no accept/decline buttons). When I send
meetings over Internet to other Outlook users I receive accept notices that
are plain emails (i.e. do not show up as an acceptance in the Outlook
meeting. How do I make my Outlook calendar interactive with other users over
Internet?