That depends on which mail support mode you are using, which you need to
specify when you post questions. Check the second line here: Help menu
If you're using Corporate Mode, you need to setup multiple MAPI profiles
here: Control Panel > Mail applet. Make sure that each user/profile uses
a different PST file (the file associated with the Personal Folder service)
If you're using Internet Mail Only mode, you need to setup OS level user
profiles (in Win9x you can set them up here: Control Panel > Password
applet > User profile tab)
Also in IMO mode if you installed Outlook before setting up user
profiles you must delete this key in the registry after you've logged in
as a 'new' user (to eliminate the profile that's copied to each new user
from the original user):
HKEY_CURRENT_USER\Software\Microsoft\Windows Messaging
Subsystem\Profiles\Microsoft Outlook Internet Settings
When you startup Outlook the first time it will recreate that profile
for a new user with a new PST file