Microsoft Outlook is one of the most widely used email and personal information management tools in business. However, like any software, it is not free from technical problems. One of the most frequent is Outlook file corruption, affecting either the PST (Personal Storage Table) or the OST (Offline Storage Table) data file. When these files become corrupted, the user may lose access to email, contacts, the calendar and other Outlook features. Consequently, significant delays and work interruptions follow. This article identifies the most common causes of Outlook data file corruption and presents effective strategies to prevent this problem and ensure continuity of work.
What an Outlook data file is and why it matters
PST and OST files: An overview
Outlook stores email messages, contacts, the calendar, tasks and other user data locally in data files. There are two main types of data file in Outlook:
- PST file (Personal Storage Table): Used for POP3 accounts (and, in older versions of Outlook, also for IMAP accounts), this file stores messages and other data locally on the user’s hard drive.
- OST file (Offline Storage Table): Used for Exchange and Microsoft 365 accounts, this file allows users to access messages and other data even when they are offline. When the user goes back online, the OST file synchronises with the server to reflect all changes.
The integrity of these files is essential for Outlook to work properly. If a data file becomes corrupted, the user may lose access to their messages and other important information, which can lead to delays and complications at work.
Common causes of Outlook data file corruption
Excessive data file size
One of the most common causes of Outlook data file corruption is excessive file size. As messages, attachments and other data accumulate over time, the PST or OST file can grow significantly. When the file becomes very large, the risk of corruption increases, especially in very old versions of Outlook, where ANSI format PST files were limited to 2 GB.
Improper shutdown and Outlook file corruption
Closing Outlook improperly or abruptly, for example during an unexpected system shutdown or a power failure, can lead to data file corruption. When Outlook does not shut down correctly, the software may not write the data files completely, resulting in damage or loss of information.
Hardware problems
Hardware failures, such as problems with the hard drive or RAM, can also corrupt Outlook data files. If the hard drive has bad sectors or the RAM is faulty, the system may not write PST or OST files correctly. As a result, errors and failures occur in Outlook.
Viruses, malware and Outlook file corruption
Virus or malware infections can corrupt Outlook data files, especially when the malware is designed to damage or manipulate email files. Therefore, maintaining adequate protection against cyber threats is essential to preserve the integrity of data files.
Strategies to prevent Outlook data file corruption
Manage data file size
One of the most effective ways to prevent Outlook data file corruption is to manage file size. Some recommended practices include:
- Archive old messages: Use the Outlook archive feature to move old messages to a separate PST file, which reduces the size of the main data file.
- Delete unnecessary messages: Regularly delete unnecessary messages, especially those with large attachments, to prevent the data file from becoming too large.
- Split the PST file: If the PST file is very large, it is advisable to split it into smaller files to reduce the risk of corruption.
Close Outlook properly
It is important to always close Outlook properly before shutting down the computer and to avoid forcing the software to close or turning off the system with Outlook open, as this can corrupt data files. If necessary, Task Manager lets you confirm that Outlook has closed completely before you shut down the computer.
Make regular backups
Keeping regular backups of Outlook data files is crucial to recover information in the event of corruption. For example, automatic backup tools can copy PST and OST files at regular intervals. If corruption occurs, you simply restore the backup, which minimises data loss and work disruption.
Keep software and hardware up to date
Keeping Outlook and the operating system up to date with the latest patches and security updates helps to avoid vulnerabilities that can lead to data file corruption. In addition, it is important to confirm that the hardware, especially the hard drive and RAM, is in good condition and free from faults. To do this, regularly use disk checking tools and hardware diagnostics to detect and fix problems before they damage data files.
Use up-to-date antivirus software
Protecting the system against viruses and malware is fundamental to prevent Outlook data file corruption. To achieve this, use reliable antivirus software, keep it up to date and run regular scans to ensure that the system is free from infections that could compromise the integrity of data files.
What to do if Outlook data files become corrupted
Use the Outlook repair tool
If an Outlook data file becomes corrupted, the first step is to use the Outlook repair tool, known as “Scanpst.exe”. This tool is part of Microsoft Office and can repair corrupted PST files (in recent versions, it does not repair OST files). Although it cannot always recover all information, the tool can fix many problems and restore access to the file.
Seek specialised technical support
If the repair tool cannot solve the problem, it is advisable to consult an IT specialist or use specialised data recovery software. Specialists have the tools and knowledge needed to try to recover corrupted data and minimise information loss.
OST files: Recreate rather than repair
OST files are a local copy of data held on the server. Therefore, when an OST file becomes corrupted, the safest solution is generally to close Outlook, rename or remove the file and let Outlook create a new one, resynchronising from the server. However, before doing so, it is advisable to confirm that there are no unsynchronised items, such as messages in the outbox or drafts created offline.
In addition, current versions of Outlook use the Unicode format by default, with a limit of 50 GB per PST or OST file. Even so, very large files slow Outlook down and increase synchronisation and repair times.
Email is not a document archive
Many problems with PST files arise because email is used as a document archive: certificates, insurance policies, fitness for work certificates and contracts are stored as attachments in each person’s mailbox. So, when the file becomes corrupted or the employee leaves the company, the information is lost. A document management platform solves this problem: documents are kept in a central repository, with metadata, expiry dates, access control and backups. Note also that Microsoft itself advises against storing PST files in network folders, because this increases the risk of corruption.
Practical example: A 40 GB shared mailbox
Consider a document management team that receives documents from contractors in a shared mailbox. Over time, the OST file reaches 40 GB and Outlook freezes several times a day. First, the team recreates the OST file and reduces the offline synchronisation period to 12 months. Next, it asks contractors to upload documents directly to the platform. Consequently, the mailbox stops growing and Outlook works normally again.
Prevention checklist
- Documents stored on the platform, not as email attachments.
- Offline synchronisation period adjusted.
- PST files kept out of network folders.
- Backups of existing PST files.
- Outlook and operating system up to date.
Frequently asked questions
Can Scanpst.exe be used on an OST file?
Not in recent versions of Outlook. In these cases, therefore, the usual solution is to recreate the OST file from the server.
Conclusion on Outlook file corruption
Outlook data file corruption can be a significant problem, leading to work interruptions and the loss of important information. However, with proactive management, including monitoring file size, closing Outlook correctly, making regular backups, updating hardware and software and protecting against viruses, it is possible to minimise the risk of corruption. If data files do become corrupted, it is essential to act quickly, using the available repair tools and, if necessary, seeking specialised support to restore data integrity and ensure continuity of work.
Read also: The impact of large files on document management platforms and File integrity in document management platforms.
For more guidance on protection against malware, see Portugal’s National Cybersecurity Centre (CNCS).