Beginning with Outlook 2003, Outlook has the capability to minimize to the tray. Right click on the Outlook icon in the tray and choose Hide when Minimized.
Minimize to the Notification tray
Using Outlook 2003 and newer, you can minimize Outlook to the notification area. Click on the tray icon and choose "Hide when Minimized" to enable (or disable) the feature.
In Windows 7 and newer, the Outlook icon may be in the overflow area, unless you changed it in Window's Taskbar properties.
Windows 7 and 8: always show the Outlook icon in the notification area, right click on the Taskbar area and choose Properties then Notification Area Customize (or go to Control panel and search for Notification Area Icons). This will open the Notification Area Icons control. Find Outlook and change it to Show icon and notifications. You'll need to double click on this icon to restore Outlook (or right click and choose Open Outlook).
If Outlook appears to close when you minimize it, check the Hide when Minimized setting.
When Outlook is set to Hide when Minimized, it won't start hidden to the notification area. You'll need to minimize it to hide it.
If you want to "tray" Outlook 2002, you need to edit the registry and set the value of MinToTray to 1.
HKEY_CURRENT_USER\Software\Microsoft\Office\10.0\Outlook\Preferences DWORD: MinToTray Value: 1
To start the registry editor, from the Start menu, Run dialog, enter regedit. That will open the registry editor. Browse down the tree until you get to Preferences under Outlook. On the right side of the screen, right click as choose New | DWORD, as shown in the screenshot. Name it MinToTray and change the value to 1 (right click on it after naming it and choose Modify)
Close outlook and reopen it for the change to take effect. Now when you minimize it, there will be an icon in the tray area. Double click on it to restore. To remove the feature, delete or rename the key.
If you don't want to edit the registry, you can use this ready-to-use registry file: