Outlook 2003 XP SP2

User1 recieves an invatation from user 2 and tries to accept and gets "
calendar cannot be found"
Me(user 3) sends an invitation to user1 and she can accept just fine
I have user2 send that same invite that will not work for use1 and I can
accept just fine

so I have a scenario that is somehow preventing user1 from accepting
invitations only from 2 (user2 and user4) other users in our whole
domain.......... and all other users in the domian can accept those same
invitations

if there is a consistancy here cant identifiy it please help