Exchange 2010 - External address resolved as an internal

Hi,

I work in an organisation who have a mixed Exchange 2003/2010 infrastructure.

Basically, i have one of my users who receive appointment from external address but when he accept the appointment, Outlook/Exchange reply to an unexisting email address.

For example : Internal user abc@abc.com receive an appointment from an external user def@def.com .

When abc accept the appointment, Outlook send the answer to def@abc.com but this address didn't exist in the organisation and not even in the local outlook contact list from the abc user.

What is strange to me, when the same user abc send an email to def, everything work properly.

Any idea ?

Thanks