While we're minimizing the Outlook, we may not want the Outlook always in the toolbar.
I checked each items in the Outlook settings but got nothing. When I searched var the web, mostly, I reached to the pages with this solution:

HKEY_CURRENT_USER \ Software \ Microsoft \ Office \ 10.0 \ Outlook \ Preferences 
New Data Type: REG_DWORD 
Value Name: MinToTray 
Data: 1 (0 to disable MinToTray)

But it seems that it takes no effect as the comments of the article.
Here is another way to minimize the Outlook to the system tray.

Open ms-outlook, the icon will appear in system tray, right click and select "hide when minimized". the next time we minimize ms-outlook it will be in system tray 

I tried it with Outlook 2007, it works well.