Came across your post when I was looking for a related SMTP issue. Since this is old, issue has prob been resolved. But if not...
Each mailbox is associated with an NT Account. Under default settings only that user can get into that mailbox. Sowhen Joe starts up Outlook (after properly logging into the NT domain)and tries to connect to his mailbox, Exchange looks to see if Joe is the owner (meaning the Exchange thinks that Joe's NT Account is linked to that mailbox).
You can pretty much change everything tho. You can change the User Account associated with Joe's mailbox, you can allow several folks to get into Joe's mail or what-have-you. Luck,
-S