Split a Large OST into Multiple PST Files During Export
Outlook OST files hold a synchronised local cache of mailbox data — emails, contacts, calendars, tasks, and notes — though depending on your Cached Exchange Mode settings, that may not include the entire mailbox history. With increased mailbox usage over time these OST files can grow to be very large. If the file size becomes too large, it can slow Outlook down, create synchronisation issues, or even crash, especially if it is at or above the size limits for Outlook. Microsoft documents separate limits for legacy ANSI-format data files (around 2 GB) and the newer Unicode format (much larger, configurable limits) — see current Outlook data-file size-limit documentation for specifics.
Exporting a large OST as several smaller PST files, rather than one large PST, is an output-management choice — it doesn't change the size of your active source OST or fix Outlook's current cache performance. Smaller PST outputs are often easier to distribute, store, open selectively, or import in stages, which matters especially for older or archived mailbox data.
EdbMails OST to PST Converter can export a readable OST into multiple PST output files while preserving supported folder hierarchy. The source OST itself is not modified or split, and recoverability depends on the condition of the source file.
| Situation | Recommended choice |
|---|---|
| Very large recovery export | Set a practical maximum size for each PST output |
| Staged transfer or import | Use multiple PSTs if the destination workflow benefits from smaller files |
| Healthy live mailbox with a large OST | Adjust the cache range or recreate the cache; PST splitting does not shrink the live OST |
| Direct Microsoft 365 or Exchange destination | Consider direct migration instead of PST staging |
How to Split Oversized OST Files to Multiple PST Files during Export
- EdbMails OST to PST Converter: You can use EdbMails OST to PST Converter. It has a built-in option to split OST files into multiple PST files in the export process, which makes it easier to manage the output files and easier to import them back to Outlook.
- Enable PST Split Option: To enable the above feature, Open the ‘Settings’ menu and select – Automatically split PST file upon reaching the size during export. Specify the maximum size of each PST file to suit your needs.
- Configure and Run the Export Process: Now that the size limit is in place, you can go ahead and do the conversion. The tool preserves the configuration and generates multiple PST files rather than one large file.
- Ensure Safe and Smooth Export: Keeping each output PST below your chosen size threshold reduces the operational inconvenience of very large files and helps make the conversion of large OST files easier to manage — though splitting does not itself guarantee against PST corruption.
Microsoft Outlook requirement
EdbMails can scan and preview the OST without the original Outlook profile. Outlook with a configured profile is required for PST export; direct migration to Microsoft 365 or Exchange does not require Outlook.
Real time scenarios of EdbMails OST file split function
- Large recovery exports: Multiple PST outputs can be easier to transfer, store, open selectively, or import in stages. This affects only the exported files and does not improve the performance of Outlook's active OST.
- Limited Storage Space: If you have limited storage space on your computer, managing large OST files can be a challenge. Splitting the exported output into multiple PST files can make individual files easier to move or store on constrained drives, but the multiple PSTs together generally require roughly the same total storage as one large file, plus some overhead — splitting doesn't reduce the total space needed, and it doesn't change your active OST's performance in Outlook.
- Enhanced data access: Breaking large OST files into multiple, more structured PST files can make it faster to locate specific data within any one file. This is a trade-off, though — spreading data across several files can also make searching or managing the full data set across all of them more complicated than working with a single archive, so it's worth weighing against your actual need.
- Defined operational limits: If an internal process requires outputs below a chosen size, configure the split threshold accordingly. File splitting alone does not establish retention, legal, or compliance completeness.
When not to split the output
Do not split merely to fix a slow live Outlook cache, reduce total storage use, prevent corruption, or simplify a data set that must be searched as one archive. Multiple PSTs add management overhead. When the target is Microsoft 365 or Exchange, direct migration can avoid PST staging altogether.
What Is Actually Being Split?
EdbMails reads the source OST and creates multiple PST output files according to the supported split setting. The source OST is not divided or reduced.
| Planning factor | Why it matters | Recommended check |
|---|---|---|
| Outlook/PST format | Older ANSI PST files and modern Unicode PST files have different practical limits. | Verify the destination Outlook version and required PST format. |
| Split size | Smaller files may simplify transfer and handling but create more outputs. | Choose a size based on the destination workflow, not an arbitrary promise. |
| Available disk space | Conversion requires room for all generated PSTs and working data. | Provide free space beyond the apparent OST size. |
| Output naming | Multiple PSTs must remain traceable to the source and sequence. | Use a consistent destination and naming convention. |
Validate Split PST Output
- Confirm all expected PST parts were created.
- Open each part in Outlook.
- Check folder continuity and representative date ranges.
- Review attachments and non-mail items.
- Retain the export log and source-to-output mapping.
For product capabilities, see the OST to PST conversion guide.
Frequently Asked Questions
Does EdbMails split the source OST file?
No. It reads the source and creates multiple PST outputs; the OST itself is not divided or modified.
What happens when a PST reaches the selected split size?
The exporter closes that output at the supported boundary and continues into another PST according to the current build's split logic.
Does splitting reduce total storage required?
No. The combined outputs generally need about the same total space as one export, plus overhead.
Does splitting fix Outlook performance?
No. It does not shrink or repair Outlook's active OST.
What PST size should I choose?
Choose a threshold that suits the destination's transfer, storage, opening, and import limits rather than assuming one universal value.
Can I migrate directly instead?
Yes. If the target is Microsoft 365 or Exchange, direct migration can avoid PST staging.
