Cache problems in Outlook

Resolving Cache Errors in Outlook

Microsoft Outlook stores a lot of data locally on the computer to enable fast and partially offline use. This includes emails, calendars, contacts, and synchronization metadata. These locally stored data are referred to as the cache.

If there are cache errors, synchronization between Outlook and the mail or groupware server can be disrupted. In practice, this manifests in various error patterns, which are often not caused server-side but affect only the local Outlook client.

This article describes typical symptoms of a cache error and explains step by step how to fix them.

Typical Signs of a Cache Error

A corrupted or inconsistent cache may be indicated by the following issues:

• Outlook constantly shows the status "Synchronization in progress..." without new emails or calendar entries appearing.
• Error messages such as "Server connection failed" or "Synchronization not possible" are displayed.
• Individual emails, folders, or appointments are missing in Outlook but are visible in webmail (e.g., OX Web App).
• Outlook responds very slowly, freezes, or crashes during synchronization.
• Calendars or contacts are no longer updated or cannot be opened.

If these symptoms occur, the cause is often a corrupted local data file or faulty synchronization information.

Possible Causes of Cache Issues

Cache errors in Outlook can have various causes, including:

• Unexpected closure of Outlook or the operating system
• Network interruptions during synchronization
• Very large mailboxes or calendars with many entries
• Faulty or incompatible add-ins
• Version changes or updates of Outlook
• Inconsistent OST or PST files

Since Outlook mirrors server-side data locally, even a small inconsistency can completely block synchronization.

Troubleshooting 1: Reset Outlook Cache by Recreating the OST File

Resetting the Outlook cache by recreating the OST file is only possible in the classic Outlook desktop version for Windows.

The most common and effective measure for cache issues is rebuilding the local Outlook data file (OST).

Procedure

1. Close Outlook completely.
2. Open the Windows Control Panel.
3. Go to Mail | Show Profiles.
4. Open the Outlook profile in use via Properties.
5. Click on Data Files.
6. Note the location of the OST file.
7. Close all windows.
8. Navigate to the OST file location in Windows Explorer.
9. Rename the file (e.g., mailbox_old.ost) or delete it.
10. Restart Outlook.

Outlook will automatically create a new OST file on the next start and resynchronize all data from the server.

Notes

• Depending on the size of the mailbox, the initial synchronization may take some time.
• During this time, Outlook may respond slowly or be partially usable.
• No server-side data is lost, as everything is downloaded again.

Troubleshooting 2: Start Outlook in Safe Mode (Check Add-ins)

Add-ins can also cause cache or synchronization problems.

Procedure to Check Add-ins

1. Close Outlook.

2. Press Windows key + R.

3. Enter the following command:

   outlook.exe /safe

4. Start Outlook.

If Outlook works without errors in safe mode, the cause is most likely an add-in.

Disabling Add-ins

Management of classic add-ins is only possible in the classic Outlook desktop version. COM add-ins are not available in web Outlook. The "Disable Add-ins" section is not valid for:

• Outlook in a browser
• Latest Outlook version
• outlook.live.com

Steps to Disable Add-ins

1. Open Outlook.
2. Go to File | Options | Add-ins.
3. Click Go… at the bottom.
4. Temporarily disable all add-ins.
5. Re-enable them one by one to identify the culprit.

Troubleshooting 3: Check Synchronization Settings

In some cases, the cache itself may be incorrectly configured.

Step-by-Step Guide

1. Open Outlook.
2. Go to File | Account Settings | Account Settings…
3. Select the affected account.
4. Click Change.
5. Check if Use Cached Exchange Mode is enabled.
6. Temporarily disable the cache and restart Outlook.
7. Re-enable the cache.

This measure also forces Outlook to reinitialize local data storage.

Troubleshooting 4: Recreate Outlook Profile

If cache errors persist, the entire Outlook profile may be corrupted.

Step-by-Step Guide to Recreate Profile

1. Open the Control Panel.
2. Select Mail | Show Profiles.
3. Click Add.
4. Create a new profile and set up the account again.
5. Set the new profile as default.
6. Start Outlook.

A new profile contains no faulty local settings and resolves many persistent issues.

Important Information

Cache errors in Outlook are common but generally a client-side problem that can be resolved without server-side intervention. The most effective measure is resetting the local data file, as Outlook will resynchronize all content from the server.

If problems persist, check add-ins or recreate the Outlook profile. Only if these steps do not resolve the issue should further analysis be considered.