New meeting request doesn't appear in calendar.

Has anybody ever come accross a new meeting request that
arrives in the users mailbox with a header of "This
meeting does not appear in the Calendar, it may have been
moved or deleted"?
This notice cannot be correct since this is a brand new
meeting request that has never been modified.

Microsoft has no articles on this type of event.

These are Oulook 2000 SP3 clients connecting to Exchange
5.5 SP4

Thanks in advance.