Why Local Email Clients Matter
Webmail, which requires an internet to access messages, local email clients like Microsoft Outlook allow users to read, write, and organize emails even when they’re offline. This is possible because of Microsoft Exchange’s Cached Exchange Mode, which stores a copy of emails locally using Offline Outlook Data Files (.OST).
The Role of OST Files in Email Storage
For Microsoft 365 (M365) and Outlook.com, OST files have become more common. These files store a cached version of Exchange data, typically containing emails from the last 12 months and reaching sizes of up to 50 GB.
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Recovering Data from OST Files: The Challenges
Unlike Personal Storage Table (.PST) files, which Outlook can open directly,
OST files are encrypted and not easily accessible. This makes recovering data tricky.
Convert OST to PST – Several third-party tools, like ost2pst.exe, help convert OST files into PST format for easier access.
Use Forensic Suites – Advanced forensic tools like AXIOM, X-Ways, FTK, and EnCase can natively parse OST files for investigation.
Beware of Duplicate Data – Since OST files sync with the Exchange server, investigators often encounter duplicate emails when analyzing both sources.
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Kindly note, sometimes orphaned OST files (files that failed to sync due to errors like mailbox corruption) can also be found on a system.
Fixing Corrupt OST Files
If an OST file gets damaged, there are a couple of ways to repair it:
scanost.exe – A built-in Outlook tool that attempts to fix corrupt OST files.
pffexport – An open-source tool (part of the libpff library) that extracts data from both OST and PST files.
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Best Tools for Viewing and Extracting Emails
While forensic suites can analyze PST and OST files, sometimes a standalone email viewer is more convenient. Some useful tools include:
XstReader – An open-source tool written in .NET/C# that allows quick access to PST, OST, and NST files.
XstExporter – A command-line tool for extracting emails and attachments in bulk.
Kernel Data Recovery Viewers – Free tools that allow users to view emails but require a paid version for exporting data.
These tools have advantages over Outlook, such as:
✅ Opening files from any Outlook version
✅ Bypassing password protection
✅ Recovering corrupted files
✅ Providing an easy-to-navigate interface
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The Reality of Free vs. Paid Email Forensic Tools
Unfortunately, when it comes to email forensics, free tools have limitations. Most investigators rely on commercial forensic suites for in-depth analysis. However, if you’re on a budget, some affordable tools include:
PST Walker – A low-cost PST viewer
Aid4Mail, Emailchemy, and Logikcull – Recommended by users for basic email extraction and analysis.
Final Thoughts
OST and PST files play a crucial role in email forensics, providing valuable insights even when data is deleted from the mail server. Whether you’re using forensic suites or standalone tools, understanding how these files work and where to find them is key to effective investigations.
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