http://office.microsoft.com/en-us/assistance/HP010416001033.aspx
"For Microsoft Exchange server and POP3 (POP3: A common protocol that is
used to retrieve e-mail messages from an Internet e-mail server.) e-mail
accounts (e-mail account: The server name, user name, password, and e-mail
address used by Outlook to connect to an e-mail service. You create the
e-mail account in Outlook by using information provided by your administrator
or Internet service provider (ISP).), a Desktop Alert is displayed only when
a new item arrives in your default Inbox. If you want to display a Desktop
Alert when an item arrives in any other Exchange folder, or when you receive
items that meet specific conditions, you must create a rule (rule: One or
more automatic actions taken on e-mail messages and meeting requests that
meet certain conditions, along with any exceptions to those conditions. Rules
are also referred to as filters.). You must also create a rule if you want to
be notified when you receive a new item in an IMAP (IMAP (Internet Message
Access Protocol): Unlike Internet e-mail protocols such as POP3, IMAP creates
folders on a server to store/organize messages for retrieval by other
computers. You can read message headers only and select which messages to
download.) e-mail account."
I still think there has to be a way to notify on a secondary inbox. I mean,
if you can do it manually by clicking "run rule on this mailbox," why can't
you automatically run the rule upon arrival?