Send As and Send On Behalf problems

Hello all,

A new problem here, so here it is :

The CEO's assistant quand Send On Behalf of the CEO email and appointments. Starting from there, it's working as well as it's supposed.

But here's the catch! After receiving an invitation from ou CEO (which was sent On Behalf from the CEO's assistant) an invited user have decided to answer by fowarding the appointment request instead of you answered it. The problem :The fowarded email got also sent On Behalf of our CEO but from the invited user.This invited user have no right on the CEO's mailbox and do not have the right to send on behalf either.

Our CEO is freaking out and we need to fix it ASAP!

Any clue?

Best regards,

Johnny

Note : Windows Server 2008 r2 - Exchange Server 2010 SP2 with RU4-v2.