How to Clear Cache in Outlook

2.2K

Caching in Outlook provides benefits such as improved performance and the ability to work with mailbox items while offline. However, in some cases, the Outlook cache can cause slow app performance (or crashes), mailbox synchronization errors, a significant increase in the size of the OST file on the user’s device, and other problems. In this case, clearing the Outlook cache may be a good starting point to resolve the issue. After clearing a corrupted or outdated cache, Outlook will download fresh data from the mailbox server.

Outlook caches different mailbox items differently, so the way how you can clear the cache depends on what you want to do.

Outlook Cache in New Outlook

The procedures we described in this article apply to classic Outlook for Windows. Note that the new Outlook for Windows uses a different sync architecture and does not expose a traditional OST cache file that can be manually removed.

If you are using the new Outlook, troubleshooting mailbox sync problems typically requires signing out of the account, resetting the app, or recreating the profile instead of deleting OST files.

Outlook AutoComplete Cache

Recipient addresses are automatically added to the AutoComplete list (also known as the nickname cache) when you send e-mail from Outlook. Thanks to this cache, when you type the recipient’s address, Outlook suggests names that are stored in the auto-complete list.

To completely clear the nickname autocompletion cache in Outlook, use one of the following methods:

• Start Outlook > go to File > Options > Mail > click Empty Auto-Complete List under Send Messages > click Yes.
  

  

• Press Win + R and run the command:
  Outlook.exe /CleanAutoCompleteCache
  

  

• Browse to the %localappdata%\Microsoft\Outlook\RoamCache folder and delete the DAR files, which names begin with Stream_Autocomplete.
  

  

Note. In older versions of Outlook 2003 and 2007, the autocomplete cache was stored in a *.NK2 file in the %APPDATA%\Microsoft\Outlook folder.

Outlook Cache Files in the RoamCache Folder

The %localappdata%\Microsoft\Outlook\RoamCache folder contains other cache files in addition to the Stream_AutoComplete *.DAT files. To clear the cache, you must delete the corresponding files:

• Stream_Calendar_* — caches calendar-related data
• Stream_CategoryList_* — information about the color categories assigned to Outlook items
• Stream_AvailabilityOptions_* – calendar availability cache
• Stream_ContactPrefs_* — Outlook contact settings cache
• Stream_ConversationPrefs_* — cached data of conversation view configurations and display options
• Stream_MRM_* — Messaging Records Management (MRM) cache
• Stream_TableViewPreviewPrefs_*
• Stream_TCPrefs — stores user-specific preferences related to the To-Do Bar and Calendar Peek features
• Stream_WorkHours_ — Outlook working hours configuration cache

Clear the Offline Address Books

Offline Address Book (OAB) files are local copies of organization address lists used by Outlook configured in Cached Exchange Mode. To delete cached versions of offline address lists, close Outlook, go to %userprofile%\AppData\Local\Microsoft\Outlook\Offline Address Books, and delete the *.OAB files.

You can also force Outlook to download a fresh Offline Address Book by navigating to Send/Receive → Download Address Book.

Outlook in Cached Exchange Mode

By default, the Outlook desktop app enables Cached Exchange Mode when connected to a mailbox on an on-premises Exchange Server or Exchange Online. In this mode, Outlook caches the contents of the Exchange mailbox in a local *.OST file on your computer. When you’re offline, you can use OST files to access mailbox e-mail items, calendar, and more.

In some cases, the size of the OST file becomes too large (tens of gigabytes), Outlook may crash, or run slowly. In this case, you can delete the local OST cache.

1. Start Outlook.
2. Click File > Account Settings > Account Settings.
3. Go to E-mail tab > click Change.
4. Uncheck the option Use Cached Exchange Mode.
   

   

5. Click Next > Finish
6. Restart Outlook
7. If you want to completely clear the local mailbox cache, you should close Outlook and manually delete the OST file from the profile folder. By default, OST files are stored in %localappdata%\Microsoft\Outlook.

This will clear the local Exchange mailbox cache. When you turn on Cached Exchange mode again, Outlook will create a new OST file and sync mailbox data from the server.

Disable the OST cache for all Outlook users via GPO

To save space in user profiles in RDS/VDI configurations, you can disable the OST cache for all Outlook users via GPO:

1. Download ADMX GPO templates for MS Office (https://www.microsoft.com/en-us/download/details.aspx?id=49030).
2. Install the ADMX templates on the AD domain controller.
3. Create a GPO and navigate to User Configuration > Policies > Administrative Templates > Microsoft Outlook 2016 > Account Settings > Exchange > Cached Exchange Mode.
4. Disable the policy Use Cached Exchange Mode for new and existing Outlook profiles.

   

5. After the policy is applied, Outlook will stop creating/using Cached Exchange Mode for affected profiles. Existing OST files may remain on disk and you can remove them manually if you no longer need them.

Recreate an Outlook Profile

If clearing the cache does not resolve mailbox sync problems, you should consider creating a new Outlook profile.

Open Control Panel > Mail > Show Profiles and create a new Outlook profile. Then config your mailbox and test whether the issue persists.

How to Clear Outlook Cache Files with PowerShell

To clear Outlook cache files with PowerShell, use the commands listed below. Note that you need to ensure that Outlook is closed before removing cache files.

Remove-Item "$env:LOCALAPPDATA\Microsoft\Outlook\RoamCache\*" -Force

Remove-Item "$env:LOCALAPPDATA\Microsoft\Outlook\Offline Address Books\*" -Recurse -Force