I had a situation recently where a 2003 client was supposed to be in cached mode but behaved as if it were in online mode.
We found that the registry had been modified and set to online mode from the registry and not from GPO or GUI.
Here is where you can check to verify.
HKCU\Software\Microsoft\Windows NT\CurrentVersion\Windows Messaging Subsystem\Profiles\ProfileName\13dbb0c8aa05101a9bb000aa002fc45a36601 holds the cached mode settings
- dword: 04 00 00 00 disables,
- dword: 84 01 00 00 enabled Cached Mode