Calendar Invitation Problem

HI,

I have two user in our compagny when they try to send an Invitation from Outlook all works OK, but the recipient receive a confirmation to accept and reply, the problem is : the reply message In question goes to  an Invalid recipients and the sender in question. But the replier receive a message : Impossible to contact the following recipient : ....  The Invalid recipients is an old employes and I cannot find any traces in Active Directory or Exchange Server, I tried to verify every group and  Forward  for those users and I found nothing.

This is my running config :
Server :
Windows 2003 SP1 and All lastest Updates
Exchange Server 2003 SP2

Clients :
Windows XP SP2 and All lastest Updates
Outlook 2003 Updated

Thanks