How to Set Outlook Sync to Skip Specific Subfolders to Save Disk

When you use Outlook with a Microsoft 365, Exchange, or Outlook.com account in Cached Exchange Mode, the entire mailbox including every subfolder is downloaded to your local hard drive. Over time, large folders like Sent Items, Archive, or old project folders can consume tens of gigabytes of disk space. This article explains how to configure Outlook to skip specific subfolders from syncing to your local machine, freeing up disk space without deleting any emails from the server.

Outlook calls this feature the Folder Sync Slider or the Download Shared Folders setting depending on your account type. By marking a subfolder as “Download Headers Only” or “Do Not Synchronize This Folder,” you stop the full content from being stored locally. The emails remain accessible online through Outlook Web App or when you switch to Online Mode.

This guide covers the steps for both Exchange accounts and IMAP accounts, plus what happens to your local data after you change the sync setting.

How Outlook Cached Exchange Mode and Subfolder Sync Work

Outlook in Cached Exchange Mode downloads a copy of your mailbox to an offline data file with the .ost extension. By default, every folder in your mailbox including subfolders is fully downloaded. The local copy allows fast offline access and reduces server load during searches.

The sync behavior is controlled by a slider in the folder Properties dialog. The slider has three positions:

• All — Download the complete folder including all items and attachments.
• Headers Only — Download only the subject, sender, and date. The message body and attachments stay on the server and are fetched on demand when you open the email.
• Do not synchronize this folder — Nothing from this folder is downloaded. The folder appears in the folder pane but shows zero local items.

For IMAP accounts, Outlook does not use the slider. Instead, sync is managed through the Send/Receive Groups dialog where you can uncheck individual folders.

Which Folders Are Safe to Skip

Folders that you rarely open in Outlook are good candidates for skipping or header-only sync. Common examples are:

• Sent Items
• Deleted Items
• Archive folders older than one year
• Large project subfolders with many attachments
• RSS Feed folders

Do not skip the Inbox, Calendar, Contacts, or Tasks folders. These are required for daily operation and offline access.

Steps to Skip Subfolders in an Exchange or Microsoft 365 Account

These steps apply to Outlook 2016, 2019, 2021, and Outlook for Microsoft 365 on Windows 10 and Windows 11.

1. Open the Folder Pane
   Press Ctrl+6 to switch to the Folder Pane view if you do not see all your mailbox folders. You can also click View > Folder Pane > Normal.
2. Right-Click the Subfolder
   Locate the subfolder you want to exclude. Right-click it and select Properties from the context menu.
3. Go to the Synchronization Tab
   In the Properties dialog, click the Synchronization tab. This tab appears only for Exchange and Microsoft 365 accounts.
4. Change the Sync Slider
   Move the slider to Headers Only or Do not synchronize this folder. If you want to keep the folder visible but remove all local data, choose the latter.
5. Confirm and Exit
   Click OK. Outlook applies the change immediately. The local .ost file shrinks during the next sync cycle.

Remove Already-Downloaded Content from a Skipped Folder

Changing the slider does not delete items that are already stored locally. To force Outlook to purge the local copy:

1. Right-click the subfolder and choose Properties.
2. Go to the Synchronization tab.
3. Click Clear Offline Items. This removes all locally cached data for that folder.
4. Click OK. The folder now shows zero items locally.

Steps to Skip Subfolders in an IMAP Account

IMAP accounts do not have the Synchronization tab. You control which folders sync through the Send/Receive Groups settings.

1. Open Send/Receive Groups
   Click the Send/Receive tab on the ribbon. Click Send/Receive Groups and then Define Send/Receive Groups.
2. Select Your IMAP Account
   In the dialog, select your IMAP account from the list and click Edit.
3. Uncheck Unwanted Folders
   In the Folder Subscription section, uncheck the subfolders you want to exclude from sync. The Inbox is checked by default and cannot be unchecked.
4. Save and Close
   Click OK to close the Edit dialog, then click Close on the Send/Receive Groups dialog. Outlook will stop syncing the unchecked folders on the next send/receive cycle.

If Outlook Still Downloads Skipped Folders

Changes Are Not Applied After Restart

If you set a folder to “Do not synchronize” but it still appears fully downloaded after restarting Outlook, the .ost file may be corrupted. Close Outlook, navigate to %localappdata%\Microsoft\Outlook, rename the .ost file to .old, and restart Outlook. A new .ost file is created. Reapply the sync settings.

Folder Slider Is Missing

The Synchronization tab only appears for Exchange, Microsoft 365, and Outlook.com accounts. If you use a POP3 or IMAP account, use the Send/Receive Groups method instead. If you use a shared mailbox, the slider is available only after you add the shared mailbox as an additional Exchange account.

Disk Space Does Not Decrease

Changing the slider prevents new items from downloading, but the .ost file does not shrink automatically. Run the Compact Now tool in Account Settings to reclaim the space. Go to File > Account Settings > Account Settings, select your Exchange account, click Change > More Settings > Advanced tab, and click Outlook Data File Settings > Compact Now.

Cached Exchange Mode vs Online Mode: Key Differences

Item	Cached Exchange Mode	Online Mode
Local storage	Downloads .ost file to local drive	No local copy; data stays on server
Offline access	Full access to cached items	No offline access
Disk space usage	Can be reduced by skipping subfolders	Minimal (only temporary files)
Search speed	Fast local search	Slower server-side search
Subfolder sync control	Per-folder slider in Properties	Not applicable

By adjusting the folder sync settings for specific subfolders, you can keep your most frequently used data available offline while reclaiming disk space from folders you rarely open. The Clear Offline Items button and the Compact Now tool help you immediately recover space that was previously used by excluded folders. For IMAP users, the Send/Receive Groups dialog provides an equivalent level of control. If you manage multiple shared mailboxes, consider switching to Online Mode for those accounts to avoid storing duplicate data on your local drive.