This error pops up when you double click ( = open ) an archived email item in outlook :
The item could not be downloaded. [OIOM]
Enable EV client logging in outlook.
In the resulting log file, you should see this error occuring:
16/11/2011 12:02:38.380: Unable to exempt PST's from PSTDisableGrow, make sure that 'PSTDisableGrowAllowAuthenticodeOverrides' registry key has been set...
This error can occur even if you have set the PSTDisableGrowAllowAuthenticodeOverrides value.
The underlying cause could be that the registry values aren't present or have been created in the registry path that is not checked by the EV client ( EV clients < 9.0.3 have this issue ).
Microsoft made it confusing to make it possible to put the Outlook policy settings in 2 different locations while EV checks only 1 of these locations.
Make sure you create(d) the PSTDisableGrowAllowAuthenticodeOverrides registry value in the following registry path:
Both locations are parsed by Outlook when it starts up, but only the first registry location is checked by the EV client.
You should set PSTDisableGrow too if it doesn't exist yet ( same location: HKEY_CURRENT_USER\Software\Policies\Microsoft\Office\14.0\Outlook\PST )