Is he logged in as himself?
I generally see this error when users are replying to e-mails from 'resource' mailboxes that they have access to. If the user is logged in as himself, and his mailbox is the primary mailbox in the outlook profile, I have no idea what could be going on.
Can he send and reply to mail through OWA? If so, I would think that there is something 'weird' setup in his Outlook profile.
From: exchangelist-bounce@xxxxxxxxxxxxx [mailto:exchangelist-bounce@xxxxxxxxxxxxx] On Behalf Of Tom Kern
Sent: Friday, September 22, 2006 2:51 PM
Subject: [ExchangeList] Strange Send on Behalf of issue
I have a user who, ever time he tries to reply to an email, Outlook 2000 and Outlook 2003 tries to populate the "From:" field with his name and then he gets an "you do not have permission to send on behalf of this user" error.
The user is himself.
It only happends when this particular user tries to reply to an email.
Composing and sending a new email works fine.
This occurs both with Outlook 2000 on Win2k and Outlook2k3 on WinXP.
Any ideas would be great.