TO field blank for sent items from Navision

We are running Navision on a W2K machine and running Exch2K on another. I recently upgraded 3 users to office 2k3. Within Navision we have a button open a new email and pull a contact address into it. All this works fine. My problem is for the three outlook 2k3 users, the TO field is blank in sent items. The email still goes thorough and arrives at the right address. This does not happen with office 97 or 2k users nor ith POP users, only those intergrated with exchange. Anyone else seen this or more importantly know how to fix it[?]