The Outlook File folder on my hard drive contains 2 of 3 email addresses I have loading into my mailbox. I have these two pointing to the one that is my Default so that all incoming messages load into its Inbox.

It appears that most things you might want to do (like pull email addresses into your Contacts) point to that folder (Outlook File) for information. The main address is the one I'd want to pull addresses from and I can't access it because it does not show up there.