OST File Too Large in Outlook? Fix the Error Safely

 

Why does Outlook say the OST file Full?

Outlook 2016, 2019, 2021, and Microsoft 365 cap the OST file at 50 GB by default. Older versions cap it at 20 GB. Once your offline cache gets close to that limit, you’ll start seeing signs of trouble:

• “The Outlook data file has reached the maximum size”
• “OST file is too large, cannot send or receive”
• Outlook freezing during sync
• Search stops working properly
• Error code 0x8004060C shows up

Common causes:

• Years of cached mail piling up when Cached Exchange Mode is set to “All”
• Large attachments adding weight fast
• Shared mailboxes cached locally
• Thousands of duplicate items created by repeated sync conflicts

Issues Caused in Outlook Due to OST File Too Large Error

Once the OST file becomes too large, error kicks in, Outlook stops behaving normally. Here’s what users typically run into:

• New emails stop syncing with the Exchange Server. You just can’t add data anymore.
• Outlook starts lagging. Small glitches turn into full pauses.
• The OST file becomes unreadable, and sometimes the OST file cannot be opened at all.
• Synchronization throws errors mid-process.
• Outlook and Exchange Server fall out of sync, and nothing you do fixes it on your own.

Manual Ways to Fix the Outlook OST File Size

Before using a professional solution, you can resolve internal error code 0000000e troubleshot OST and try Outlook’s built-in methods to reduce the size of an oversized OST file. These approaches are free to use and may temporarily improve Outlook’s performance if the mailbox has recently grown in size.

1. Reduce the Cached Exchange Mode sync slider (File > Account Settings > Change) from “All” to 12 or 6 months.
2. Empty Deleted Items and Junk, then compact the file (Data File Properties > Advanced > Compact Now).
3. Use Mailbox Cleanup to find items over 5 MB and archive them.
4. Raise the size limit via the MaxLargeFileSize registry value — risky, because oversized OST files corrupt more easily and slow Outlook down.

The problem: none of these actually gets your data out of the OST. The moment the profile breaks, the mailbox is deleted from the server, or the OST gets orphaned, that file becomes unreadable by Outlook — and the data inside it is stuck.

Why is Converting OST to PST the Permanent Fix?

A PST file is portable and importable into any Outlook profile. Converting your oversized OST to PST lets you:

• Fix issues like errors have been detected in Outlook.ost
• Free the OST to rebuild it fresh and small
• Keep a complete, importable archive of every email, contact, calendar, and task
• Split the output into multiple small PSTs so you never hit the size wall again

Outlook has no built-in “export OST to PST” option for orphaned or inaccessible OST files, and Microsoft’s own utilities don’t cover this scenario. That’s where a dedicated converter earns its price.

Fix OST File Too Large Error with OST to PST Converter

Manual tricks only buy you time. The real fix is getting your data out of the OST entirely, and that’s where a dedicated tool helps. The Microsoft OST to PST Converter handles this without size limitations, so you can convert even a heavily oversized OST file without running into the same wall again.

• Converts OST files to PST format in bulk, along with emails, contacts, calendars, tasks, and notes
• Splits the resultant PST file so you’re not stuck with one giant file again
• Removes duplicate emails during conversion to keep your mailbox clean
• Works without needing Outlook installed or connected to Exchange Server
• Lets you preview mailbox items before conversion, so nothing gets missed
• Supports date filtering, in case you only want to convert a specific range
• Works with orphaned OST files too, useful if the Exchange profile is already broken

Working Guide to Resolve Outlook OST Too Large Error:

1. Download the Microsoft OST to PST Converter Tool on your PC.

2. Then, browse & select File(s) or Folder to add the Ost files >> click Proceed.

3. Now, review & select the OST files and press Proceed.
4. After that, go to Export File Format & select Outlook Data File(.pst).

5. Lastly, apply required filters & click Start Export.

Aryson vs OSTScan.exe and other third-party tools

People searching “ostscan.exe” are usually looking for an OST equivalent of ScanPST.exe. Two things to know: Microsoft’s Inbox Repair Tool (ScanPST.exe) is for PST files, and unofficial “OSTScan” downloads floating around are unverified executables — running a random .exe against your only copy of a 50 GB mailbox is a gamble. Repair tools also don’t fix the Outlook OST file size problem; even a repaired OST is still an oversized OST.

Capability	Aryson OST to PST Converter	OSTScan.exe / repair utilities	Typical free converters
Opens orphaned/inaccessible OST	Yes	Partial, unreliable	Rarely
Duplicate removal during conversion	Yes	No	No
Metadata & folder structure preserved	Yes		Frequently lossy
Export to Office 365 / Live Exchange	Yes	No	No
Free preview before purchase	Yes (50 items/folder demo)	No	Varies
Handles 50 GB+ files	Yes	Crashes common	Size-capped

Why users trust Aryson

Aryson Technologies has been building Outlook and email recovery tools since 2014, with a product line covering the full data-recovery stack — OST recovery, PST split/merge, and cloud backup. Support runs 24×7 via live chat, so a stuck conversion at 2 a.m. before a Monday deadline isn’t a dead end.

For enterprise Outlook environments

If Outlook is your organization’s primary client, the “OST too large” error is never a one-machine event — it’s a fleet problem that hits your heaviest users first (executives, legal, support desks with shared mailboxes). What that changes:

• Standardize the fix, don’t improvise per ticket: Set a conversion SOP: preview the OST, dedupe, split at 10 GB, archive PSTs to your file server or export straight into the user’s Office 365 mailbox. Helpdesk closes these tickets in one pass instead of registry-hacking each machine.
• Offboarding and legal hold: Departed-employee OSTs are recurring liabilities. A converter that opens orphaned files with metadata intact turns each one into a defensible, importable archive — timestamps and recipient fields preserved for eDiscovery.
• Prevention policy: Pair the fix with a Cached Exchange Mode GPO (3–12 month sync window) so rebuilt OSTs don’t become oversized again. The conversion handles the backlog; the policy stops the regrowth.
• Licensing: Per-machine licenses are one-time with free updates to verify the current licensing model and whether a corporate/technician license tier exists — if yes, name it here; enterprise buyers look for it , so scoping cost against ticket volume is straightforward.

Moving Mailboxes Between Microsoft 365 Organizations?

If your oversized OST file is the result of a Microsoft 365 tenant migration, company merger, business acquisition, or domain change, exporting individual OST files to PST may not be the most efficient approach.

Conclusion

The OST file too large error can significantly affect Outlook’s performance, synchronization, and access to mailbox data. Although manual methods such as reducing the Cached Exchange Mode sync range, compacting the OST file, and archiving older emails can temporarily reduce the file size, they do not permanently resolve the issue. Converting the OST file to PST is a more reliable long-term solution, as it preserves mailbox data, maintains folder hierarchy, and makes the mailbox easier to access and manage in the future.

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