Exchange Out of Office – Sending Replies for Every Email

I would have the users open their primary mailbox and then open the "shared" mailboxes as secondary mailboxes. Microsoft gives you the click-for-click procedure here: http://support.microsoft.com/kb/291626 Since you've already got these people opening the shared mailboxes you shouldn't run into permission issues,

If you need these people to be able to "Send As" the shared mailboxes, grant them "Send As" permission (or, more appropriately, create a group, put them in the group, then grant the group "Send As" permission) on the shared mailbox recipients.

Using "Public Folders" to do this is a very nice option, but that feature is going away in future versions of Exchange I'm not directing people toward it as much as I used to. We used to do a lot of this kind of thing for, for example, incoming customer service emails. We'd have the incoming issues go into one public folder, a CSR would pick up the issue email and move it to a "work in process" public folder (to let the other CSRs know that someone "had the issue") and then they'd move it into a "Resolved" folder, along with the email they sent as a resolution (they'd cc the "Resolved" folder) for history.

I had a similar problem once when a user had been moved to a different Exchange cluster, but Outlook had cached the previous reference and kept trying to send to the previous cluster, even when I specifically selected the user from the Global Address list.

The way I fixed it was in Outlook, start typing the user's name until it auto-suggests the username, use the arrow key to select the name in the auto-suggest list and hit Delete. Try typing the name again and it shouldn't auto-detect it, so you can then select the user from the GAL.