What is the Outlook Cached Mode Sync Slider? (Complete Guide)

The Cached Mode Sync Slider – also known as 'Mail to keep offline' – is an Outlook setting that controls the amount of mailbox data is downloaded and stored locally in an OST file. It was introduced in Outlook 2013. It lets you control how much of your mailbox data is to be stored on your computer, when the Cached Exchange Mode is enabled. This feature creates a local copy - OST file - of your mailbox, so you can access emails and all other mailbox items (calendars, contacts etc.) without internet. Cached Exchange Mode also downloads the Offline Address Book (OAB), so contact lookups and name resolution work without a server connection.

However, if “Mail to Keep Offline” slider is set to “All mail”, Outlook will download all your mailbox data to the local cache. Over time, this makes your OST file very large - especially if you have years of email data. A large OST file can affect your email client’s performance, cause longer load times, and lead to sync-related issues.

The sync slider helps manage OST file growth by setting a time-based cutoff: emails older than the selected period are kept on the server but not stored locally. This reduces your mailbox size and improves you application’s performance. Email outside the sync windows safely remain on the server and you can access them any time via Outlook Web App – OWA. Read further to know how it helps manage the size of an OST file and optimize Outlook performance.

Sync Slider Settings - What Each Level Does

The following table shows a general comparison of each sync slider setting and its impact on your local storage. 

How to Change the Sync Slider in Outlook?

Below, we have mentioned the methods to change the Sync Slider settings in Outlook:

Method 1 - Use Outlook Account Settings  

To adjust the sync slider in Outlook 2013, 2016, 2019, or Outlook 365, follow these steps:

• Launch Outlook application.
• Go to the File menu.
• Select Account Settings > Account Settings.

• Select your account.
• Then, click on Change. 

• Adjust the Mail to keep offline slider (e.g., 1 or 3 months).

• Click Next and then Finish.

After you change the setting, Outlook will start updating your local cache (OST file) in the background. It removes or adds older items based on your new setting.

Method 2 - Use Registry Editor (IT Admins)

You can use Windows Registry Editor to adjust the sync slider programmatically or for a specific user profile. Here’s how you can do this:

• Press Windows + R keys.
• Type regedit and press Enter.
• Approve the UAC prompt if it appears.

• Go to:

HKEY_CURRENT_USER\Software\Policies\Microsoft\Office\Vxx.0\Outlook\Cached Mode

• If the Cached Mode folder doesn’t exist, right‑click the Outlook key.
• Choose New > Key.
• Name it Cached Mode.
• Create a DWORD (32‑bit) Value named syncwindowsetting.
• Enter a value in months:

• 1 → 1 month
• 3 → 3 months
• 6 → 6 months
• 12 → 1 year
• 24 → 2 years
• 36 → 3 years

• Close Registry Editor.
• Restart Outlook.

This allows day‑level precision:

• 3 → 3 days
• 7 → 1 week
• 14 → 2 weeks

See the table mentioned below to know the proper combinations of syncwindowsetting and syncwindowsettingdays values for each duration. 

In Outlook 2016 and later the sync window is set to 12 months by default, when Cached Exchange Mode is first enabled.

Method 3 - Use Group Policy (Enterprises)

In larger enterprise environments, you can manage the Cached Mode Sync Slider centrally through Group Policy. Once you apply this setting, the slider in Outlook is locked (greyed out) and users cannot change it. Follow these instructions:

• Download the latest Microsoft 365 Apps ADMX template.

• Open the Group Policy Management Console – GPMC.
• Import the downloaded ADMX template.
• In GPMC, go to:

• User Configuration → Administrative Templates → Microsoft Outlook → Account Settings → Exchange → Cached Exchange Mode.

• Locate Cached Exchange Mode Sync Slider.
• Enable it.
• Set the desired sync duration (e.g., 1 month, 3 months, 1 year, or All mail).
• Link the Group Policy Object – GPO - to the relevant Organizational Unit (OU).

Once applied, all users in that OU will have the sync slider locked to the value you set.

For details, see Learn Microsoft Cache Exchange Mode article

Why the Sync Slider Setting Matters for OST File Health?

The default size limit of OST is a maximum of 50 GB in Outlook 2010 and later versions. The default location of Outlook OST file is: %LOCALAPPDATA%\Microsoft\Outlook\ on Windows.

However, you may start encountering issues when it grows beyond 20–25 GB, such as:

• Slow or incomplete search results
• Outlook freezing when switching folders
• Delays in sending/receiving emails
• Outlook crashing

Setting the “All Mail” option downloads your entire mailbox data, which increases your OST file size. If your OST file grows too large, it becomes susceptible to corruption. 

So, it is suggested to keep only 1 to 6 months of email available offline. It ensures smooth performance, reduces corruption risk, and prevents common OST‑related issues.

How to Further Limit OST Sync beyond the Slider?

There are other options in Outlook that you can use to limit the amount of locally stored data. These options are especially helpful if you work with shared mailboxes, public folders, or large amounts of mailbox data.

Option A. Disable Shared Folder Synchronization

By default, Outlook downloads data from shared mailboxes and shared folders that you have access to. If these shared folders contain a large amount of emails, they can significantly increase the size of your OST file. Here’s how to do so:

• Launch Outlook.
• Click File > Account Settings > Account Settings.

• Choose your Exchange account.
• Click Change.

• Then, click More Settings.

• Click the Advanced tab.
• Disable Download Shared Folder option.

• Click OK and then Next.
• Click Finish.

Once these settings are disabled, Outlook will stop storing shared mailbox and public folder data in the local OST file. This can significantly reduce OST size and improve Outlook performance, especially in environments where users have access to multiple shared mailboxes.

Option B. Folder-level Sync Filter

With this, you can control which emails and mailbox items Outlook will keep in each folder. It helps you to avoid unnecessary storage of old or unused content. Follow the instructions below: 

• Right‑click on any mail folder (e.g., Inbox).
• Choose Properties.

• Go to the Synchronization tab.

• Click Filter.
• Set criteria to limit cached items (e.g., only unread messages or items from the last 30 days).

• Click OK.

Outlook will apply the filter during the next sync cycle.

This option is useful if certain folders, like newsletters or archives, do not need to be fully cached.

Sync Slider not Working? - Common Issues and Fixes

The table below helps you to identify the possible cause for Sync Slider not working issue and the appropriate solution to fix it.

When the Sync Slider is not Enough

The Cached Mode Sync Slider helps control OST file size and improves Outlook performance. However, it cannot resolve every issue related to your Outlook data file. For example: if your OST file becomes inaccessible due to an unexpected shutdown, Outlook crash, disk issue, synchronization problem or any other internal/external factor, you cannot restore and access your mailbox data by adjusting the sync slider.  In such a case, you may require a dedicated OST converter tool, such as Stellar Converter for OST. This tool can convert any inaccessible or orphaned OST file to PST without any size limit, and then you can import it into Outlook. This tool also supports exporting mailbox data directly to Microsoft 365, or a live Exchange Server mailbox.  

Conclusion

The Cached Mode Sync Slider helps you to control how much mailbox data your Outlook stores in the local OST file. By limiting the amount of cached data, you can reduce OST file size and improve your application performance. However, if the OST file becomes inaccessible, Stellar Converter for OST can help by converting the OST to PST format. It allows you to recover and access your mailbox data.