"Parameters value is not valid" message

Outlook217
We are using Outlook 2000 and when we send a meeting

request to our clients, they get an error message that

says something like "...Parameters value is not valid."

and it does not add it to their calendar. They said that

when other people send them outlook appointments, they can

just drop the email into their calendar and it adds it to

their calendar. What might we have set up wrong that

causes this?


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