I built an open-source, offline OST to PST converter for Windows

K

Kirin

Member
OS Version(s)
  1. Windows
Outlook version
Outlook 365 64 bit
Operating system::    Windows 11 26H2
Outlook version:     2609 Build 16.0.2043
Email type or host:    Microsoft 365
I’ve been working on OST2PST, a C++ tool that reads an Outlook OST and creates a Unicode PST without requiring Outlook, MAPI, an account, or network access. It includes a portable Windows GUI and a CLI.

In one recorded run, it converted a 26.25 GB OST to a 22.98 GB PST in about 32 minutes, with no skipped items and internal verification passing. I’ve also opened and checked mail and attachments from a smaller converted sample in Outlook. Compatibility across more OST files and Outlook versions still needs testing.

The project is MIT licensed. I’d welcome feedback, especially reproducible issues from people testing with copies of their OST files: GitHub - Hyper-Beast/OST2PST: Offline OST to PST converter for Windows with a native GUI
 
Kirin said:
tool that reads an Outlook OST and creates a Unicode PST without requiring Outlook
Click to expand...
I did a quick test on an Orphaned OST, everything works as advertised for email, Calendar, and Contacts. Nice!
 
Das klingt nach einem sehr nützlichen Werkzeug, besonders für verwaiste OST-Dateien, bei denen kein ursprüngliches Outlook-Profil mehr verfügbar ist. Für einen breiteren Praxistest wären Angaben zu verschlüsselten Nachrichten, wiederkehrenden Kalendereinträgen und sehr großen Anhängen hilfreich. Eine automatisch erzeugte Zusammenfassung mit Anzahl der konvertierten Ordner, Elemente und Fehler würde die Kontrolle nach der Konvertierung zusätzlich erleichtern.
 
If you're looking for an OST to PST converter, WholeClear OST to PST Converter is worth considering. It helps users convert OST mailbox data into PST format for use with Outlook. Before choosing a tool, check whether it supports orphaned OST files, preserves email attachments and folder structure, and handles large mailbox files efficiently. Always test with a small sample first to verify the conversion results.
 
Kirin said:
I’ve been working on OST2PST, a C++ tool that reads an Outlook OST and creates a Unicode PST without requiring Outlook, MAPI, an account, or network access. It includes a portable Windows GUI and a CLI.

In one recorded run, it converted a 26.25 GB OST to a 22.98 GB PST in about 32 minutes, with no skipped items and internal verification passing. I’ve also opened and checked mail and attachments from a smaller converted sample in Outlook. Compatibility across more OST files and Outlook versions still needs testing.

The project is MIT licensed. I’d welcome feedback, especially reproducible issues from people testing with copies of their OST files: GitHub - Hyper-Beast/OST2PST: Offline OST to PST converter for Windows with a native GUI
Click to expand...
MIT Faculty retired prof. here - - can this be used to condense the OST so as to conform to MIT's new adhence to reduced OneCloud size? (2GB) although reduction to BU's new adhence to reduced GDrive size (15 GB) would be better appreciated? - Both educational institutions ported mgt and cost of mail storage!!
 
StatsMavem said:
Hi Professor,

Thank you for your question and interest in the tool!

To answer directly: No, this tool (nor any software) can physically compress an existing oversized OST down to 2GB while keeping all emails inside. However, it is precisely the tool you need to solve your MIT 2GB quota problem.

Here is the technical reality and how you can use this tool to achieve your goal safely:

1. Why "Compacting" the OST Won't Work​

  • OST is a cloud mirror: Your local .ost file is purely an offline cache reflecting what resides in your Microsoft 365 / Exchange mailbox. As long as those historical emails remain on the server, the data payload exists.
  • Outlook’s native compacting: It only reclaims fragmented white space after emails have already been deleted. It cannot magically shrink existing emails and attachments.
  • The quota is server-side: Even if you restrict the local sync slider to keep only the past 1 month locally, your server mailbox is still over quota, and MIT will block your sending/receiving.

2. How to Use This Tool to Conform to MIT's 2GB Limit​

Instead of trying to shrink the OST in place, the working workflow is Offline Archiving:

  1. Convert your OST to PST (Fully Offline):
    • Close Outlook completely so the .ost file is not locked.
    • Run this tool to parse your local .ost file and convert your entire historical archive into a standalone .pst file on your local drive.
    • (Advantage: Unlike Outlook's built-in export, which frequently freezes or crashes on large multi-GB mailboxes, this tool parses the file directly and offline without MAPI or cloud dependencies).
  2. Mount the Archive in Outlook:
    • Open Outlook, go to File -> Open & Export -> Open Outlook Data File, and select the generated .pst.
    • All your historical folders, threads, and attachments remain 100% accessible and searchable locally.
  3. Purge the Cloud Mailbox:
    • Once you verify that all folders and emails are readable inside the mounted PST, permanently delete older emails from your active account (and empty both Deleted Items and Recoverable Items).
    • Your active cloud mailbox will immediately drop below the 2GB threshold.

3. Critical Safety Warning: Protect Your PST​

Because the emails are moved out of MIT's redundant cloud storage, the local PST becomes your sole surviving copy.

  • Do not place active PSTs in OneDrive / Google Drive: Microsoft explicitly warns against mounting a PST directly inside a real-time sync folder (OneDrive, Dropbox, Google Drive), as file locking conflicts can easily corrupt the database header.
  • Follow the 3-2-1 Backup Rule:
    • Keep 3 copies across 2 different storage media (e.g., your internal drive and an external portable SSD/HDD).
    • Always close Outlook before copying the .pst file for cold backups.
I hope this helps you navigate MIT's new storage policy without losing decades of correspondence! Let me know if you run into any issues during the conversion.
Click to expand...
 
StatsMavem said:
conform to MIT's new adhence to reduced OneCloud size? (2GB) although reduction to BU's new adhence to reduced GDrive size (15 GB) would be better appreciated
Click to expand...
Just to throw another option in the mix - If you just want a copy of your stuff, that only you control, local off-line storage? e.g. Export old stuff to a PST, delete it from the server, mount (Open) the local PST in Outlook. Or, if you won't always have access to an Outlook client, something like Free MailStore Home.
 
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Rule on a shared mailbox only runs when my Outlook is open >
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