Outlook calendar changes send messages to other pc’s inbox, help!

At a client of ours there is a problem with outlook / exchange.

two of the pcs at the client is being funny… Everytime they add something to their calendar / change it – it sends it in a message to another PC in the office.

So two pc’s are sending messages to another PC on any calendar updates / changtes – even though the recipient of these messages dont have anything remotely to do with the calendar entries.

How do I fix this?