Outlook Performance Issues When There Are Too Many Items or Folders in Cached Mode .OST or .PST File

If your mailbox has more than 10,000 folders, 100,000 mail items in a mail folder, or 5,000 items per calendar, you may experience the following issues or errors in Outlook 2021, 2019, 2016, 2013, or 2010.

  • Delay in sending or receiving emails
  • Synchronization issues or errors
  • Slow search results
  • Missing mail items or folders
  • Outlook takes more time to load
  • Delay in meeting update and sync issues with Shared and delegated calendars

Important Notes:

  • Outlook creates OST only when your Exchange or Office 365 email profile is set up in Offline Mode or Cached Exchange Mode.
  • You may also experience the above-mentioned issues if the OST file size exceeds the recommended (~20-25 GB) limit, irrespective of the number of mail items or folders.

Methods to Reduce Mail Folders or Items in Mailbox in Cached Exchange Mode

To reduce the number of mail folders or mail items in the mailbox, you can move them to a local PST file. For this, you can use the Outlook Archive option, Import/Export wizard, or drag/drop the mail folders and items to a local PST file. Below we have discussed these methods in detail.

Archive Mail Items

Outlook provides an option to archive mail items based on a specific date. All mail items sent or received before the selected date are archived in a local PST file from your default OST file. This helps reduce the number of mail items and folders from the mailbox and OST file.

You may either manually archive the items or enable the auto-archive feature. The steps are as follows:

  • In Outlook, navigate to File > Info > Cleanup Tools > Archive or File > Info > Tools > Clean up old items.
  • Click Archive this folder and all subfolders and select the folder you want to archive.
  • Select a date under Archive items older than.
  • By choosing different dates, you can create multiple archives or .pst files. You can also choose to keep the mail items in certain folders, such as the Sent folder, longer than other items in your other mail folders.

archive mail items to local pst file

  • Check the Include items with "Do not AutoArchive" checkbox to archive all items.
  • Click OK.

Apply Retention Policy

The retention policy allows Outlook users to remove mail items or messages from the mailbox and local OST after a certain time. Administrators can define the retention policy or Outlook users may also apply retention policies on mail folders to automatically remove messages from the mailbox and move them to the Archive. The messages remain accessible in the Archive.

To set a retention policy on email folders, follow these steps:

  • Open the web browser and sign in to your email account using your organization's Outlook Web App URL.
  • Enter your username and password. Click Sign in.
  • In the folder pane, right-click the message or folder for which you want to apply a policy and select Assign Policy.

apply retention policy

  • Choose the Archive Policy or retention policy under the Labels you want to assign to the mail folder and all mail items in it.

You may also use the Outlook client to choose the Retention Policy.

 choose retention policy time

Tip: If you want a message to adhere to the same policy as the folder it is in, select Use parent folder policy.

Adjust Sync Settings

When your email profile is set up in default Cached Exchange Mode, Outlook provides a cache mode sync slider setting to adjust the number of mail items or folders to store in the OST file. You can adjust this slider to avoid storing or exceeding the number of mail folders or items. Follow these steps:

  • Open Outlook and go to File > Info > Account Settings > Account Settings.
  • Choose your email profile and click Change.
  • Under Offline Settings, adjust the Mail to keep the offline If it's currently set to All, move it to 3 years or lower to reduce the number of mail items, calendar items, and mail folders in your offline OST file.

adjust the sync slider in cached exchange mode

  • Once adjusted, click Next and then click OK.
  • Restart Outlook for the changes to take effect.

This will synchronize the number of mail items sent or received per the sync slider setting.

You can also edit registry keys to adjust the number of months or years of data you want to synchronize for shared calendars. The steps are as follows:

  • Press Windows+R, type Regedit, and press the Enter
  • Navigate to the following location:

HKEY_CURRENT_USER\Software\Microsoft\Office\16.0\Outlook\Cached Mode

  • Create or modify the CalendarSyncWindowSetting value to 1 (Primary Calendar Folder) or 2 (All Calendar folder) based on your need to enable the Calendar sync window.
  • Next, create or modify the CalendarSyncWindowSettingMonths registry with values 1, 3, 6, or 12 (in months). This will help control the number of months in the Calendar Sync window, reducing the number of items in the calendar.

Clear Offline Items

Clearing the Offline Items may also help reduce the number of mail folders and items from your local OST file. This can improve the Outlook performance and avoid issues or errors due to too many mail items or a large OST file size. To do so, follow these steps:

  • Find and confirm the folders having too many mail items or folders.
    • Right-click on the mail folder and choose Properties.
    • Click the Synchronization tab and check the number of items the folder contains.

clean or clear the offline mail items

  • Once you have found and confirmed the folders having too many mail items, click the General tab and then click Clear Offline Items.

clear offline items from inbox

  • This will remove the offline mail items and help reduce the number of mail items and OST file size.

data removed from the ost file after clearing offline items

  • Click OK and restart Outlook.

You may also open the Calendar properties to Clear Offline Items from the Calendar folder.

Compact OST File

Compacting the OST file removes the white space from the file left behind by the deleted mail items or folders. Thus, it does not reduce the mail items or folders but can help lower the OST file size, which may improve the Outlook performance.

The steps are as follows:

  • Close Outlook and open Control Panel.
  • Search and click on the Mail
  • Click Data Files…
  • Select the OST associated with your profile and click Settings.
  • Click Advanced and then click Outlook Data File Settings…

compact the ost file

  • Click Compact Now. This compresses and reduces the OST file size.

To Wrap Up

To avoid Outlook performance issues, keep your mail items and mail folders below the maximum limit and OST file size under the recommended limit. You can follow the methods discussed in this guide to reduce the mail folders and items. In addition, you can use a third-party OST to PST converter tool, such as Stellar Converter for OST, to reduce mail items, folders, calendar items, and the OST file size. We recommend the software as Outlook may crash or freeze when you archive, move mail items, or export them to a PST file from an oversized OST or profile having too many mail items or folders.

The software can split a large or oversized OST file into smaller PST files based on size, mail folder, or date range. You can then open these PST files in Outlook to view and access the mail items without worrying about the Outlook performance.

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Ravi Singh linkdin Icon

Senior Writer at Stellar®. He is an expert Tech Explainer.

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