The ultimate open-source utility to merge, combine, and join multiple Outlook PST files into a single master file.
Completely free, no size limits, and enterprise-grade reliability.
Are you struggling with scattered Outlook data files? PST Merge Tool is a powerful Windows application designed to consolidate unlimited .pst files into one managed archive. Unlike paid alternatives, this tool is 100% free and open source.
It was built specifically to handle large mailboxes (up to 2TB) and includes a registry fix to bypass the default 50GB Outlook file size limit. Perfect for IT administrators performing backup recovery or users organizing years of emails.
- ✅ Merge unlimited PST files into one master PST
- ✅ 2TB size limit support (removes the default 50GB Outlook restriction)
- ✅ Preserves folder structure (Inbox, Sent Items, subfolders, etc.)
- ✅ Handles all item types (Emails, Contacts, Calendar, Tasks, Notes)
- ✅ Graceful cancellation - stop safely without corrupting data
- ✅ Persistent logging - every action saved to a log file
- ✅ Disk space validation - warns before running out of space
- ✅ Error resilience - continues processing even if individual items fail
- Windows 10/11 or Windows Server
- Microsoft Outlook installed (uses Outlook Interop)
- .NET Framework 4.5 or later
- Download the latest release from Releases
- Run
PstMerger.exe - Click "Fix PST Size Limits" to remove the 50GB restriction (one-time setup)
- Select source folder containing your PST files
- Select destination for the merged PST
- Click "Start Merge" and wait for completion
⚠️ Important: Close Outlook before running the merge. Do not use the computer for heavy tasks during the merge process.
Found a bug?
This project is open source and we welcome your feedback!
Please Submit an Issue if you encounter any problems or have feature ideas.
# Clone the repository
git clone https://github.com/YourUsername/PstMergeTool.git
cd PstMergeTool
# Build with MSBuild
msbuild PstMerger.csproj /p:Configuration=ReleaseThe compiled executable will be in bin\Release\PstMerger.exe.
The tool uses Microsoft Outlook Interop to:
- Open each source PST file
- Recursively traverse all folders
- Copy items to the destination PST, preserving folder structure
- Safely close and detach each PST after processing
This ensures maximum compatibility and data integrity, as Outlook handles all the low-level PST format details.
By default, Outlook limits PST files to ~50GB. The "Fix PST Size Limits" button modifies:
HKEY_CURRENT_USER\Software\Microsoft\Office\15.0\Outlook\PST
HKEY_CURRENT_USER\Software\Microsoft\Office\16.0\Outlook\PST
Setting MaxLargeFileSize and WarnLargeFileSize to ~2TB.
MIT License - see LICENSE file.
Mithun
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