How to configure Outlook for first session using cached mode and subsequent sessions in on-line mode RRS feed

  • General discussion

  • How to configure Outlook for first session using cached mode and subsequent sessions in on-line mode

    This will go over the steps for a second Outlook session to get online mode.


    What this mode will do: for the first session it will use the Office 365 VHD with OST and Search caching. The environment this is targeted at will generally not have concurrent sessions for the same user. But if it happens, then the second session will not connect to the VHD and will simply operate Outlook in online mode assuming that Outlook is run in this second (concurrent) session. Online mode is a fallback position if there is a second session concurrently with the first session which already has the OST in use. The first session will always get the VHD attached and have the cached OST and Search.


    1. Use cached mode in Outlook.

    To do this, all user sessions need to have the correct Microsoft GPOs applied which configure for Outlook:  1) Cached Mode and 2) the length of time for the cache to contain emails. These admin templates (.admx files) are available through Microsoft. This is very important for this values to be configured for this process to work and recover if a second session is set to online mode. FSLogix does not set Cached Mode or Length of Time to include in cache. The MS GPO (or other means; registry, etc.) must be used to set these values.



    2. Add registry values

    The FSLogix Office 365 container (registry values as HKLM\Software\Policies\FSLogix\ODFC or through the GPO) needs to be enabled and correctly configured. The two values below control this:

     1. OutlookCachedMode

     2. VHDAccessMode.

    Their default values are what should be used, respectively 1 and 0








     If set to '1' and the O365 Container is successfully attached, the Outlook setting that enables cached mode will be temporarily set until the container is detached. This can be used to ensure that cached mode is only used when the container is attached.




     A value of '0' (default) means normal direct access behavior.


     3. Update: If FSLogix Profiles are being used, then the OutlookCachedMode value on the Profiles key (not the ODFC key as that is noted above) must be set to 0 (zero). This is a new setting which did not exist when this document was written and may be removed in the future. But for now (10 March 2020) it needs to be set to 0 (zero) for this process to work correctly.

     4. Finally, in order for this feature to work, Outlook must be configured to default to online mode. This is not an FSLogix setting, but a Microsoft setting and thus in not in the FSLogix .admx file.

         HKEY_LOCAL_MACHINE\Software\Policies\Microsoft\office\16.0\Outlook\OST\NoOST:DWORD = 2


    What this does is if the Office 365 container attaches properly, FSLogix will override this value (using FSLogix set value rules) on a per user basis to 0 and Outlook will use the GPO settings where the OST and search will be cached.



    3How this Operates

    If the VHD does not attach because the Office 365 container is already in use then the value of 2 for NoOST will NOT be overridden and the user will be placed in online mode with Outlook and everything should work that way for that session. The first session will continue to use Cached Mode and when the second session is closed there will be no residual effects on Cached Mode for any first or second session.

    [NOTE: the reason that the GPO is required (per FSLogix testing) is that if the value of 2 for the NoOST reg value is not overridden and the profile saves this, then Outlook will not go back into cached mode because the registry keys for cached mode and length of cache get changed if NoOST=2 is actually seen by Outlook. Thus the need for the GPO to keep these settings always correct.]


    Tuesday, June 25, 2019 9:15 PM

All replies

  • Hi Micah,

    IT is a useful article and thank you so much. I have a question, I am only configuring the Profile container with Outlook 2019 but I need to accomplished same behaviour. Please advice me what are the thing do I need to configure in order to achieved same. 

    Thank you.



    Wednesday, March 18, 2020 1:21 PM