Hi Mark,
I am having a similar problem with Outlook 2007 after changing to Win 7. This is what I wrote on another forum but no takers yet:
Outlook 2007 &Win 7.
I am having problems sending emails to some addresses and get this message from "System Administrator":
Your message did not reach some or all of the intended recipients.

Subject: FW: Election
Sent: 07/02/2010 16:54

The following recipient(s) cannot be reached:

Where does this come from?
Not Norton because it does it with NIS uninstalled.
Not ISP because it doesn't happen on my XP laptop.
It's really frustrating because my correspondence is now split between my PC and my laptop.
P.S. It tries all three of the accounts I have on Outlook and the failure message comes back to a different account to that which sent it.