I have one user who get the above NDR error when sending to all addresses, internal or external. She is using Outlook 2010 on Windows 7 Enterprise. The body of the error in the NDR is this: You can't send a message on behalf of this user unless you have permission to do so. Please make sure you're sending on behalf of the correct sender, or request the necessary permission. If the problem continues, please contact your helpdesk. Diagnostic information for administrators: Generating server: #MSEXCH:MSExchangeIS:/DC=com/DC=:[578:0x000004DC:0x0000001D] #SMTP# She admits that she was doing "something" this morning, but she isn't sure what (quelle surprise)... She can log onto another machine and use Outlook fine, and OWA works fine as well, so it must be a local setting. I have deleted her MAPI profile and deleted C:\Users\\AppData\Local\Microsoft\Outlook, but without joy. I removed her local profile (They don't user roaming profiles), and still the same issue. I have repaired outlook, but no joy either. This makes me think that she has somehow made a global change in outlook, but I can't figure out what it could be. Any ideas anyone??