cdo scheduled task runs as user1 but not user2 ?

[Roger]

I've got this small vba code

Public Sub sendmail()
Dim osession As Object

Set osession = CreateObject("MAPI.Session")
osession.logon "npescan", "", False, True, 0, False
... read stuff...
osession.logoff
set osession = nothing
end sub

I'm using the CDO.DLL version found with EXCHANGE5.5 SP 4
the npescan profile has an internet mail account and a personal folder
developed on winXP

the function runs fine from an ms-access IDE session
the function runs fine as a scheduled task if I run it as "xxx\user1"
where user1 is a local administrator
but the function fails at the 'logon' line if I run it as "xxx\user2"
where user2 is both a local administrator and a domain administrator

any idea why ?

if I move this to win2000
ide runs fine
both user1 & user2 (set up per the XP environment) both fail

in all cases, the failure is MAPI_E_LOGON_FAILED

do I need a different cdo.dll for win2K ?