Free PST Merge Tool for Outlook

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.

Overview

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.

Features

• ✅ 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)
• ✅ Duplicate detection (optional - can be disabled for faster processing)
• ✅ 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

Requirements

• Windows 10/11 or Windows Server
• Microsoft Outlook installed (uses Outlook Interop)
• .NET Framework 4.5 or later

Usage

1. Download the latest release from Releases
2. Run PstMerger.exe
3. Click "Fix PST Size Limits" to remove the 50GB restriction (one-time setup)
4. Select source folder containing your PST files
5. Select destination for the merged PST
6. Click "Start Merge" and wait for completion

Command Line Options

The tool supports the following command line parameter:

• --skip-duplicates or -s: Skip duplicate checking during merge (faster processing, may include duplicates)
  • Example: PstMerger.exe --skip-duplicates
  • When enabled, the window title will show "(Duplicate checking: DISABLED)"

⚠️ Important: Close Outlook before running the merge. Do not use the computer for heavy tasks during the merge process.

Screenshots

Support & Feedback

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.

Building from Source

## Cleanup
rm -rf bin/ obj/

# Clone the repository
git clone https://github.com/YourUsername/PstMergeTool.git
cd PstMergeTool

# Build with MSBuild
msbuild PstMerger.csproj /tv:4.0 /p:TargetFrameworkVersion=v4.0 /p:Configuration=Release

The compiled executable will be in bin\Release\PstMerger.exe.

How It Works

The tool uses Microsoft Outlook Interop to:

1. Open each source PST file
2. Recursively traverse all folders
3. Copy items to the destination PST, preserving folder structure
4. 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.

Registry Fix 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.

Outlook Repair Tool

Location C:\Program Files\Microsoft Office\root\Office16\SCANPST.EXE

Instructions

License

MIT License - see LICENSE file.

Author

Mithun

© 2026 All Rights Reserved.