How to Split Large OST Files to Improve Outlook Performance

Microsoft Outlook is a cornerstone of professional communication, relied upon by millions for email management, calendar organization, and task coordination. However, as mailboxes grow with emails, attachments, and other data, the Offline Storage Table (OST) files that store this information can become excessively large, leading to performance issues such as slow loading times, synchronization errors, and even application crashes. Splitting large OST files into smaller, more manageable segments is a proven strategy to enhance Outlook’s efficiency, reduce the risk of data corruption, and ensure seamless operation. This comprehensive guide explores the reasons behind oversized OST files, their impact on Outlook performance, and detailed methods to split them effectively, with both manual and automated solutions.

Understanding OST Files and Their Role in Outlook

OST files, or Offline Storage Table files, are local data files created by Microsoft Outlook when configured with Exchange Server, IMAP, or Microsoft 365 accounts. These files store a synchronized copy of mailbox data, including emails, contacts, calendars, tasks, and notes, allowing users to access their information offline. When an internet connection is restored, changes made offline sync with the server, ensuring data consistency. However, as users send and receive emails, especially those with large attachments, the size of OST files can grow significantly, impacting Outlook’s performance.

Why Do OST Files Grow Large?

Several factors contribute to the growth of large OST files:

• Heavy Email Traffic: Frequent sending and receiving of emails, particularly in professional environments like customer support or sales, increases OST file size.
• Large Attachments: Emails with high-resolution images, videos, or large documents accumulate data quickly.
• Lack of Cleanup: Failure to delete old or unnecessary emails, especially from the Deleted Items or Sent Items folders, contributes to file bloat.
• Shared Folders and Mailboxes: Accessing shared mailboxes or public folders caches additional data locally, expanding the OST file.
• Extended Sync Settings: Outlook settings that retain emails for long periods without archiving or deletion lead to unchecked growth.

Consequences of Oversized OST Files

When OST files exceed their optimal size, they can cause a range of issues that degrade Outlook’s performance:

• Slow Performance: Large files slow down email loading, folder switching, and search functions.
• Synchronization Errors: Oversized files may fail to sync properly with the Exchange Server, leading to missing emails or delayed updates.
• Application Crashes: Outlook may freeze or crash when processing large OST files, disrupting workflows.
• Data Corruption: Large files are more prone to corruption, risking data loss.
• Storage Issues: Excessive OST file sizes consume significant disk space, potentially triggering low-storage warnings.

The maximum size limit for OST files varies by Outlook version:

• Outlook 2007: Up to 20 GB
• Outlook 2010 and later: Up to 50 GB (extendable via registry settings)

Exceeding these limits amplifies the risks, making it critical to manage OST file size proactively.

Why Splitting Large OST Files is Essential

Splitting large OST files into smaller, more manageable parts offers multiple benefits that directly enhance Outlook’s performance:

• Improved Speed: Smaller files reduce the time required to load emails, switch folders, and perform searches.
• Reduced Corruption Risk: Smaller files are less susceptible to corruption, safeguarding critical data.
• Easier Management: Splitting files by size, date, or folder makes it easier to organize and back up data.
• Better Synchronization: Smaller OST files sync more efficiently with the Exchange Server, minimizing errors.
• Storage Optimization: Dividing large files frees up disk space, improving overall system performance.

By implementing effective splitting strategies, users can maintain a smooth and reliable Outlook experience.

Manual Methods to Split Large OST Files

Several manual methods can help reduce the size of large OST files, improving Outlook performance without requiring third-party tools. While these methods are effective for minor size reductions, they may have limitations, particularly for severely oversized or corrupted files.

Method 1: Archive Old Emails to Reduce OST File Size

Archiving old emails moves them from the active OST file to a separate archive file, reducing the primary file’s size. This method is ideal for users with large volumes of historical data.

Steps to Archive Emails in Outlook:

1. Open Outlook and navigate to File > Tools > Clean Up Old Items.
2. Select the folder to archive or choose the entire mailbox.
3. Specify a date for the Archive items older than option to move older emails.
4. Click Browse to choose a save location for the archive file (typically a PST file).
5. Click OK to create the archive and reduce the OST file size.
6. Repeat for multiple folders or date ranges to create additional archives.

Benefits:

• Frees up space in the primary OST file.
• Creates accessible backups for future reference.
• Reduces synchronization load on the Exchange Server.

Limitations:

• Archiving does not split the OST file into multiple parts; it only moves data.
• Manual archiving can be time-consuming for large mailboxes.
• Archived files may still require management to prevent future size issues.

Method 2: Compact OST Files Using Outlook’s Built-In Feature

Compacting an OST file removes unused space (white space) created by deleted items, reducing the file size without deleting data.

Steps to Compact an OST File:

1. Open Outlook and go to File > Account Settings > Account Settings.
2. In the Data Files tab, select the OST file to compact.
3. Click Settings, then navigate to the Advanced tab.
4. Click Outlook Data File Settings and select Compact Now.
5. Allow the process to complete, which may take several minutes depending on the file size.

Benefits:

• Reduces file size without data loss.
• Built-in feature requires no additional software.
• Improves Outlook’s performance for moderately large files.

Limitations:

• Compacting only works if there is significant white space (typically >20%).
• Does not address severe size issues or split files into multiple parts.
• Ineffective for corrupted OST files.

Method 3: Delete Unnecessary Emails and Attachments

Removing redundant or unnecessary emails and attachments directly reduces the OST file size.

Steps to Delete Emails and Attachments:

1. Open Outlook and sort emails by size using the Search box (type messagesize:>5mb to find large emails).
2. Select and delete large or unnecessary emails.
3. Right-click the Deleted Items folder and select Empty Folder to permanently remove deleted items.
4. Save large attachments to your computer and remove them from emails to further reduce size.
5. Regularly clear the Junk Email and Sent Items folders.

Benefits:

• Directly reduces OST file size by removing unneeded data.
• Simple and quick for small-scale cleanups.
• Improves search and loading performance.

Limitations:

• Permanent deletion risks losing important data.
• Time-consuming for large mailboxes with many emails.
• Does not split the OST file into smaller segments.

Method 4: Disable Shared Folder Synchronization

Shared folders, such as those from shared mailboxes or public folders, can significantly increase OST file size. Disabling their synchronization reduces the data cached locally.

Steps to Disable Shared Folder Sync:

1. Open Outlook and go to File > Account Settings > Account Settings.
2. Select the OST file under the Data Files tab and click Settings.
3. In the Advanced tab, uncheck Download shared folders.
4. Click OK and restart Outlook to apply changes.

Benefits:

• Reduces OST file size by limiting cached data.
• Improves synchronization performance.
• Easy to implement without data loss.

Limitations:

• Disables offline access to shared folders.
• May not significantly reduce size for users with minimal shared folder data.
• Does not split the OST file.

Method 5: Adjust Offline Mail Sync Settings

Outlook’s offline mail sync settings determine how much email data is stored locally. Reducing the sync period can shrink the OST file.

Steps to Adjust Sync Settings:

1. Open Outlook and go to File > Account Settings > Account Settings.
2. Select the Exchange account and click Change.
3. Move the Mail to keep offline slider to a shorter period (e.g., 3 months or 6 months).
4. Click Next, then Finish, and restart Outlook.

Benefits:

• Reduces OST file size by limiting synced data.
• Maintains access to recent emails while clearing older ones.
• Improves synchronization and performance.

Limitations:

• Older emails become inaccessible offline.
• Does not split the OST file into multiple parts.
• May require frequent adjustments for active mailboxes.

Limitations of Manual Methods

While manual methods can reduce OST file size, they have significant drawbacks:

• Time-Consuming: Sorting, archiving, or deleting emails manually is labor-intensive for large mailboxes.
• Risk of Data Loss: Deleting emails or attachments may result in losing important data.
• Limited Splitting Capability: Manual methods do not split OST files into multiple smaller files, which is often necessary for optimal performance.
• Ineffective for Corruption: Manual methods cannot address corrupted OST files, which require specialized tools.
• Technical Complexity: Some methods, like adjusting sync settings, may be challenging for non-technical users.

For users with severely oversized or corrupted OST files, automated tools offer a more efficient and reliable solution.

Automated Tools to Split Large OST Files

Professional OST splitter tools are designed to split large OST files into smaller PST files based on criteria like size, date, folder, or sender. These tools overcome the limitations of manual methods by automating the process, preserving data integrity, and handling corrupted files.

Why Use an OST Splitter Tool?

• Efficiency: Splits large files quickly without manual intervention.
• Flexibility: Allows splitting by size (e.g., 1 GB, 5 GB), date, folder, or sender.
• Data Integrity: Preserves email properties (To, Cc, Bcc, attachments) and folder hierarchy.
• Corruption Handling: Repairs and splits corrupted OST files.
• User-Friendly: Intuitive interfaces make the process accessible to all users.

Recommended OST Splitter Tools

Several reputable tools are available to split large OST files. Below are some of the best options, with detailed steps for one of the most popular tools, Aryson OST Splitter.

Aryson OST Splitter

Aryson OST Splitter is a robust tool that splits oversized OST files into smaller PST files with high accuracy. It supports splitting by size, date, sender, or folder and is compatible with all Outlook versions.

Steps to Split OST Files with Aryson OST Splitter:

1. Download and install Aryson OST Splitter on your Windows PC.
2. Launch the software and click Add File or Add Folder to select the oversized OST file.
3. Verify the selected file and click Next.
4. Choose the folders to split and select a splitting option (e.g., by size, date, or sender).
5. Specify the desired size for output PST files (e.g., 1 GB, 5 GB).
6. Apply advanced filters, such as excluding duplicate emails or setting a password for output files.
7. Select a destination folder for the split PST files.
8. Click Split to initiate the process and monitor the progress.
9. Once complete, access the split PST files in Outlook.

Features:

• Splits OST files into multiple PST files without data loss.
• Supports batch splitting for multiple files.
• Offers advanced filters for selective splitting.
• Compatible with Windows 11, 10, and earlier versions.
• Provides a preview of OST file contents before splitting.

Benefits:

• Automates the splitting process, saving time.
• Reduces the risk of corruption in output files.
• Maintains email formatting and hierarchy.
• Handles corrupted OST files effectively.

Other Notable Tools

• Kernel for OST to PST Converter: Splits OST files by size (1 GB to 45 GB) and supports multiple output formats (PST, MSG, EML). It also offers cloud migration options.
• SysInfo OST Splitter: Allows splitting by size, date, sender, or folder, with features like duplicate email removal and password protection.
• Recoveryfix for OST to PST Converter: Converts and splits OST files into smaller PSTs, with support for corrupted files and multiple saving options.
• BitRecover OST Splitter: Offers advanced splitting options and maintains email properties during the process.

Choosing the Right Tool

When selecting an OST splitter tool, consider the following:

• Compatibility: Ensure the tool supports your Outlook version and Windows OS.
• Splitting Options: Look for flexibility in splitting criteria (size, date, folder).
• Ease of Use: Choose a tool with an intuitive interface for non-technical users.
• Data Integrity: Verify that the tool preserves email properties and folder structure.
• Trial Version: Opt for tools with a demo version to test functionality before purchasing.

Best Practices to Prevent Large OST Files

Preventing OST files from becoming oversized is as important as splitting them. Implement these best practices to maintain optimal file size and Outlook performance:

• Regularly Archive Emails: Use Outlook’s AutoArchive feature to move old emails to PST files periodically.
• Delete Unnecessary Data: Clear the Deleted Items, Junk Email, and Sent Items folders regularly.
• Save Attachments Externally: Download large attachments to your computer and remove them from emails.
• Limit Offline Sync: Adjust sync settings to store only recent emails offline (e.g., 3 or 6 months).
• Monitor File Size: Check OST file size periodically via File > Account Settings > Data Files to identify growth trends.
• Use Professional Tools: Employ OST splitter tools proactively to split files before they reach critical sizes.

Troubleshooting Common Issues During OST Splitting

Even with manual or automated methods, users may encounter issues when splitting large OST files. Below are common problems and solutions:

• Corrupted OST File: If the OST file is corrupted, manual methods will fail. Use a professional tool like Aryson OST Splitter to repair and split the file.
• Incomplete Splitting: Ensure the tool has sufficient disk space and is not interrupted during the process.
• Outlook Crashes During Manual Methods: Close unnecessary applications to free up system resources and try again.
• Slow Splitting Process: For very large files, splitting may take time. Use a tool with batch processing to streamline the task.
• Access Denied Errors: Run the splitter tool as an administrator and ensure the OST file is not in use by Outlook.

Recommendations for Optimal Outlook Performance

To maximize Outlook’s efficiency and prevent issues related to large OST files, follow these recommendations:

• Combine Manual and Automated Methods: Use manual methods like archiving and compacting for regular maintenance, and employ OST splitter tools for significant size reductions.
• Backup Before Splitting: Always create a backup of the OST file before splitting to prevent data loss.
• Update Outlook: Ensure you’re using the latest version of Outlook to benefit from improved file handling and performance.
• Monitor System Resources: Upgrade RAM or storage if your system struggles with large OST files.
• Schedule Regular Maintenance: Set a recurring schedule (e.g., monthly) to check and manage OST file size.

Conclusion

Splitting large OST files is a critical step to improve Outlook performance, reduce the risk of data corruption, and ensure seamless email management. Manual methods like archiving, compacting, and deleting unnecessary emails offer quick solutions for minor size reductions, but they fall short for severely oversized or corrupted files. Professional OST splitter tools, such as Aryson OST Splitter, provide a reliable, efficient, and flexible way to split files into smaller PSTs, preserving data integrity and enhancing Outlook’s speed. By combining these methods with proactive maintenance practices, users can maintain a smooth and efficient Outlook experience.

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Suggestions and Recommendations

• Proactive Management: Regularly monitor OST file size and implement archiving or splitting before files reach critical limits.
• Use Trusted Tools: Invest in reputable OST splitter software to ensure data safety and efficiency.
• Educate Users: Train team members on Outlook maintenance practices to prevent oversized files in shared environments.
• Automate Processes: Use tools with batch processing and scheduling features to streamline OST file management.
• Consult Experts: For complex mailboxes or persistent issues, seek assistance from IT professionals or software support teams.

Frequently Asked Questions (FAQs)

1. What is an OST file in Outlook?
   An OST file is an Offline Storage Table file that stores a local copy of mailbox data from Exchange Server, IMAP, or Microsoft 365 accounts, allowing offline access.
2. Why do OST files become large?
   OST files grow due to heavy email traffic, large attachments, lack of cleanup, shared folder syncing, or extended offline sync settings.
3. How do large OST files affect Outlook performance?
   Large OST files cause slow loading, synchronization errors, application crashes, and increased risk of data corruption.
4. What is the maximum size of an OST file?
   The maximum size is 20 GB for Outlook 2007 and 50 GB for Outlook 2010 and later, though it can be extended via registry settings.
5. Can I split an OST file manually?
   Yes, manual methods like archiving, compacting, or deleting emails can reduce OST file size, but they don’t split files into multiple parts.
6. What are the benefits of splitting OST files?
   Splitting improves Outlook speed, reduces corruption risk, optimizes storage, and enhances synchronization efficiency.
7. Are there risks to splitting OST files manually?
   Manual methods risk data loss, are time-consuming, and may not work for corrupted files.
8. How do I check the size of my OST file?
   Go to File > Account Settings > Data Files in Outlook to view the size of your OST file.
9. What is the best tool to split large OST files?
   Aryson OST Splitter is highly recommended for its flexibility, data integrity, and ease of use.
10. Can I split a corrupted OST file?
    Yes, professional tools like Aryson OST Splitter can repair and split corrupted OST files.
11. How long does it take to split an OST file?
    The time depends on the file size and tool used, ranging from minutes for small files to hours for very large ones.
12. Will splitting an OST file affect my data?
    Using a professional tool ensures data integrity, preserving email properties and folder structure.
13. Can I split OST files by specific criteria?
    Yes, tools like Aryson OST Splitter allow splitting by size, date, sender, or folder.
14. Do I need Outlook installed to split OST files?
    No, most professional tools work independently of Outlook.
15. How can I prevent OST files from becoming large?
    Regularly archive emails, delete unnecessary data, limit offline sync, and use OST splitter tools proactively.