Fixing the "Unable to activate send-on-behalf-of list" Error

This MS Exchange Team Blog article (as well as this follow-up) go into great detail about the history of this notorious error, but erroneously claims that it only occurs in muli-domain environments.  We have the issue here, and we have a single-domain forest.

The short version is that Exchange Server 2003 SP2 introduced changes in the security model for the delegate-setting process which, combined with updates on the client end, result in the following error when a user attempts to modify delegates:

The Delegates settings were not saved correctly. Unable to activate send-on-behalf-of list. You do not have sufficient permission to perform this operation on this object.

This error does not occur for all mailbox, and we've found neither rhyme nor reason for when it does, but there is a fix.

• For Outlook 2003 users apply this patch, then perform the registry modifications described here.
• For Outlook 2007 users apply this patch, then perform the registry modifications described here.