You want to enable Cached Exchange Mode for shared mailboxes in Outlook while keeping your primary mailbox in Online Mode. This configuration reduces server load and improves performance when accessing large shared mailboxes, but Outlook does not offer a direct toggle for this. The default settings apply the same mode to all mailboxes in a profile.
This article explains how to override the default behavior using a registry key that forces shared mailboxes to download a cached copy. You will learn the exact steps to modify the Windows Registry, the prerequisites required, and the limitations of this workaround.
Following these steps will let your primary mailbox stay online while shared mailboxes use a local OST file for faster access and offline availability.
Key Takeaways: Enable Cached Exchange Mode for Shared Mailboxes Only
- Registry key at HKEY_CURRENT_USER\Software\Microsoft\Exchange\AlwaysDownloadSharedFolders: Forces Outlook to download shared mailbox data into a local OST file even when the primary mailbox is in Online Mode.
- Outlook must start once after creating the key: The registry change takes effect on the next Outlook launch, not immediately after modification.
- Only works with Exchange Online or Exchange Server 2013 and later: Older Exchange versions and non-Microsoft email accounts do not support this registry override.
How Outlook Applies Cached Exchange Mode to Shared Mailboxes
Outlook does not separate caching behavior by mailbox type in its graphical interface. When you set a profile to Cached Exchange Mode, every mailbox added to that profile downloads a local OST file. When you set the profile to Online Mode, no mailbox downloads an OST file. This all-or-nothing approach creates a problem for users who want the speed of cached access for shared mailboxes but need to keep their primary mailbox online to avoid sync delays or disk space issues.
The registry key AlwaysDownloadSharedFolders was introduced in Outlook 2013 and remains supported in Outlook 2016, Outlook 2019, Outlook for Microsoft 365, and the new Outlook for Windows. When this DWORD value is set to 1, Outlook downloads shared mailbox folders into a local OST file while leaving the primary mailbox in its configured mode. If the primary mailbox is set to Online Mode, the shared mailboxes still get cached. If the primary mailbox is set to Cached Exchange Mode, the shared mailboxes are also cached, but the key has no visible effect because they were already downloading.
This feature relies on the Exchange Autodiscover service to locate the shared mailbox. The shared mailbox must have a valid Exchange license or be a shared mailbox object in Exchange Online or on-premises Exchange Server 2013 or later. Mailboxes from third-party providers such as POP, IMAP, or Google Workspace are not affected by this registry key.
Steps to Enable Cached Exchange Mode for Shared Mailboxes Only
Before you begin, close Outlook completely. You must have administrator rights on the Windows computer to edit the registry. If you are on a corporate-managed device, your IT department may restrict registry changes or deploy this setting via Group Policy.
- Open Registry Editor
Press Windows + R to open the Run dialog. Type regedit and press Enter. If the User Account Control prompt appears, click Yes. - Navigate to the Exchange key
In Registry Editor, go to the following path:HKEY_CURRENT_USER\Software\Microsoft\Exchange
If the Exchange key does not exist, right-click the Microsoft key, select New > Key, and name it Exchange. - Create the AlwaysDownloadSharedFolders value
Right-click the Exchange key in the left pane. Select New > DWORD (32-bit) Value. Name the new value AlwaysDownloadSharedFolders. Double-click the value, set the data to 1, and click OK. - Set your primary mailbox to Online Mode
Open Outlook. Go to File > Account Settings > Account Settings. Select your primary email account and click Change. Under Offline Settings, uncheck Use Cached Exchange Mode. Click Next and then Finish. Restart Outlook. - Verify that shared mailboxes are cached
After Outlook restarts, open a shared mailbox. In the folder pane, right-click the shared mailbox name and select Data File Properties. If the General tab shows a file path ending in .ost, the shared mailbox is using Cached Exchange Mode. The primary mailbox will not have an OST file associated with it.
What to Do If Shared Mailboxes Are Still Not Cached
Outlook continues to show shared mailboxes in Online Mode
The most common cause is that the registry key was created under the wrong hive or with incorrect spelling. Double-check that the key is named AlwaysDownloadSharedFolders exactly, with no spaces. Confirm that the data value is 1 and that the key is located under HKEY_CURRENT_USER\Software\Microsoft\Exchange. If you created the key under HKEY_LOCAL_MACHINE, move it to the per-user location.
The shared mailbox does not appear in the folder pane at all
You must manually add the shared mailbox to your Outlook profile before caching can occur. Go to File > Account Settings > Account Settings. Select your primary account, click Change, then More Settings. On the Advanced tab, click Add under Open these additional mailboxes. Type the shared mailbox name or email address and click OK. Restart Outlook.
Outlook crashes or becomes slow after enabling the registry key
If the shared mailbox contains a very large number of items, the initial OST download can be resource-intensive. Let Outlook run overnight to complete the sync. If crashes persist, delete the registry key by setting AlwaysDownloadSharedFolders to 0 or removing the value entirely. Then run Outlook in safe mode by holding the Ctrl key while clicking the Outlook icon. This disables add-ins that may conflict with the caching process.
The registry key is missing after a Group Policy update
If your organization uses Group Policy to manage Outlook settings, a policy refresh may overwrite the registry key. You can check the applied policies by running gpresult /h gpresult.html in Command Prompt as administrator and reviewing the HTML report. Work with your IT department to deploy the AlwaysDownloadSharedFolders setting via an ADMX administrative template if available.
Online Mode vs Cached Exchange Mode for Shared Mailboxes: Key Differences
| Item | Online Mode | Cached Exchange Mode with AlwaysDownloadSharedFolders |
|---|---|---|
| Description | Shared mailbox data is retrieved directly from the Exchange server with every request | A local OST file stores a copy of the shared mailbox data for offline access and faster browsing |
| Offline access | Not available | Full offline access to previously synced folders |
| Server load | Higher due to repeated requests for folder contents and metadata | Lower after initial sync because Outlook reads from the local OST file |
| Disk space usage | None | Uses disk space proportional to the shared mailbox size |
| Search performance | Search queries run against the Exchange server, slower for large mailboxes | Local Windows Search index provides instant results |
| Sync delay | None; data is always current | Potential delay of a few minutes depending on the sync frequency |
| Supported Exchange versions | All Exchange versions | Exchange Server 2013 and later, Exchange Online |
You can now configure Outlook to cache only the shared mailboxes you need while keeping your primary mailbox in Online Mode. This setup reduces server requests for large shared folders and gives you offline access to their contents. After verifying the registry key works, test the behavior with a low-priority shared mailbox first to confirm stability. For advanced management, consider using the Outlook Group Policy administrative template to deploy the AlwaysDownloadSharedFolders setting across multiple machines.