Outlook 2016 Secondary Mailbox Cached Mode

After migrating to Outlook 2016 from 2010, I noticed this inconsistency.

If you use secondary mailboxes in Outlook, you’re probably going to want them in Online Mode rather than Cached Mode. With Cached Mode on, you’ll have an OST file created for each extra mailbox you add, and you’ll hit performance issues if you have over 500 folders over all mailboxes added to the account.

One of the ways to avoid these performance issues is turning off ‘Download shared folders’ in the mailbox settings:

‘Download shared folders’ disabled

This can be done manually, or company wide with the Group Policy setting “Disable shared mail folder caching” found in User Configuration / Administrative Templates / Microsoft Outlook 2016 / Outlook Options / Delegates. Enabling this will disable and grey out the option as per the screenshot above.

However, I was previously doing this through a registry setting ‘CacheOthersMail’ under HKEY_CURRENT_USER\Software\Policies\Microsoft\office\16.0\outlook\cached mode with the value set to 0. This worked on Outlook 2010 fine I believe, but in 2016 it did something slightly strange. Although clicking on a secondary mailbox’s folders showed they were in Online Mode with the status bar status of ‘Online’, the ‘Download shared folders’ tickbox was still enabled. I’ve confirmed this on both CTR and MSI versions of Office 2016.

At first I thought nothing of this, as it seemed to be working as intended. However, after a while I worked out that having it configured this way lead to performance issues, and people who had over 500 folders had cases where the inbox would stop updating. Changing the tickbox setting resolved the issue, despite the secondary mailboxes before and after this showing as ‘Online’. I didn’t dig into this any further so I can’t explain what was actually going on, but at a guess it was still doing some sort of sync or connection on each folder despite it being in Online Mode.

My advice is – make sure the ‘Download shared folders’ tickbox is off rather than just checking that the folders show as being ‘Online’. If you really need a secondary mailbox in cached mode but want to disable it by default, you could add it as a seperate mailbox account which will have it’s own cached mode settings.



12 thoughts on “Outlook 2016 Secondary Mailbox Cached Mode

  1. I am using the latest ADMX and making the change as noted above but it does not seem to create the reg key and the checked box is still ticked to cache shared folders. I am running Office 365 ProPlus (Click to Run installed version).

    1. CTR uses a different registry path altogether – I haven’t tested, but check something like HKEY_CURRENT_USER\SOFTWARE\Microsoft\Office\ClickToRun\REGISTRY\MACHINE\Software\Policies\Wow6432Node\Microsoft\Office\

      Search for a registry hive of ‘ClickToRun’ and you’ll hopefully find it. Please report back!

  2. I noticed this as well. I had assumed that using the registry method would automatically untick the option. However it still remains. I wanted to avoid having to set this option every time I setup a new user.

  3. I had the same problem. I fixed it by adding Dword ‘DownloadSharedFolders’ with value of 0 in the same [HKEY_CURRENT_USER\Software\Policies\Microsoft\office\16.0\outlook\cached mode] key.
    The purpose of ‘DownloadSharedFolders’ set to 0 is to grey it out so the user cannot change it. It also appears to reinforce the ‘CacheOthersMail’ setting. No documentation on the Interwebs helped.

      I couldn’t find DownloadSharedFolders property anywhere, not even after manually setting that option.

      I spent at least 4 hours of googling but none of the articles I found were mentioning this property.

      THANKS A LOT!!!! Great contribution!

  4. Caching Shared Folders is dangerous. It may speed things up in certain scenarios but I just had an executive assistant lose a lot of calendar data that was only in the cache for the executive. Disabling it is highly recommended. In this scenario, the user had two shared mailboxes the executives and a team mailbox. The size of our mailboxes is extensive and the cache slider was set to 1 year.

    The symptom reported was the user noticed the executives inbox hadn’t updated in days. So we turned off the Shared Folders. That resulted in a flushing of the OST file and apparently the users updates to the executives calendar also had not been synchronizing with the 365 mailbox. Resulting a loss of all the changes the user made to the executives calendar for an undetermined length of time. A Royal SNAFU of EPIC Proportions.

    I’ve also witnessed similar issues with a large team mailbox where a user drags and drops an email to a folder and the email disappears as the cache isn’t updated.

    My assumption is that Outlook times out silently while scanning a very large OST and then fails to update the actual mailbox from the cache. Things only get worse from there. The data the user sees is no longer the source of truth.

    If you want to turn off Shared Folders make sure you backup the OST and perhaps go into offline mode and export the mailboxes to PST files before you do it. Or at least print the calendars.

    1. I have hit some similar issues with a primary mailbox being in cached mode and not syncing stuff, but it’s rare. Agree, ideally just leave shared mailboxes in Online mode! Thanks for sharing your experience.

  5. GPO Path is wrong. Instead of User Configuration / Administrative Templates / Microsoft Outlook 2016 / Outlook Options / Delegates

    this should be

    User Configuration > Administrative Templates > Microsoft Outlook 2016 > Outlook Options > Delegates > Disabled Shared Folder Caching

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