Outlook 2007 - RPC over HTTP - Send AS fails.

Windows Vista - Outlook 2007 - RPC over HTTP - SEND AS FAILS.

I have a vista machine the is running Outlook 2007 configured to an exchange 2003 sp2 server. All functionality is working as expected except for when the user attempts to utilize the "send as" for another mailbox. The user does have send as and send on behalf rights for the other user and the other user's mailbox is also opened in this instance of outlook.

An underliverable message is returned:

Your message did not reach some or all of the intended recipients.

Subject: FROM USER
Sent: 6/7/2007 8:34 AM

The following recipient(s) could not be reached:

RECEPIENT on 6/7/2007 8:29 AM
You do not have permission to send to this recipient. For assistance, contact your system administrator.
MSEXCH:MSExchangeIS:/DC=internal/DC=DOMAIN:SERVER

The same message is returned even if we put in the email address of the Default user. (send as self)

The entire process works as expected on an XP professional Outlook 2003 box using RPC over HTTP configured with the very same accounts.

Thanks in advance for any suggestions.