I've never had outlook notify me so I'm afraid I can't help with making that notify you 100% of the time but what I can tell you is what I use.
I use a freeware product installed called
POPPeeper. You install this little program and set it up with your mailbox information and pick how often you want it to check (mine is set for every 10 minutes) and then it goes out and checks for mail. It can play a sound when it finds new mail. Also, it shows the number of new messages in the system tray or an empty mailbox icon when it is 0. You can even preview the messages by clicking on the icon or right-click and pick "run email client" and it will open Outlook for you. What is really nice is this is not limited to one email address, it checks my account I use with Outlook but it also checks all my internet mailboxes as well.
NOTE: In the General options make sure "run POP peeper at windows startup" & "close button will minimize POP peeper
are set so that it is always running to go check for mail at your specified intervals.