You're launching Outlook from a script or from the command-line?
If you're not using VBScript then this is OT.
If you are then:
Outlook is running in CW or IMO mode?
CW: You're not authenticated through your profile, as you should be, if
you're being requested for authentication. This will happen if you're
logged into one domain and your email using authentication in another
domain, even through a trust. You have to authenticate through the profile
when it is created to eliminate the authentication request.
IMO: You're being prompted for authentication because you chose not to save
this information in your account settings.
This information is distributed in the hope that it will be useful, but
without any warranty; without even the implied warranty of merchantability
or fitness for a particular purpose.