This was a bit of a puzzle at first:
Outlook was prompting for credentials for the two add-in mailboxes but not for the primary mailbox.
It took a few minutes of working through the settings to figure out that there was one very important thing missed during the initial setup of this mailbox:
The Connect to Microsoft Exchange using HTTP was not enabled and our RPC/HTTPS Outlook Anywhere settings were not in place!
Fortunately, the missed setting was on the laptop I am working on, but if it belonged to a client that was trying to use their new laptop setup there could have been a frustrated phone call.
Microsoft Small Business Specialists
Co-Author: SBS 2008 Blueprint Book