If you use Outlook with a Microsoft 365 or Exchange account, your offline OST file automatically grows as you receive and send email. When that file approaches 100 GB, Outlook may stop synchronizing, display errors, or refuse to open. This hard limit exists because the Windows Search indexer and Outlook’s database engine cannot reliably manage a file larger than 100 GB. This article explains why the 100 GB limit exists and gives you step-by-step methods to split your mail across archive files so you keep your OST file under the limit.
Key Takeaways: Managing the Outlook 100 GB OST Limit
- File > Account Settings > Account Settings > Data Files: Locate your current OST file and check its size in the folder location.
- File > Account Settings > Account Settings > Data Files > Add: Create a new Outlook Data File (.pst) to serve as an archive destination.
- File > Info > Cleanup Tools > Archive: Move or copy old items to the archive PST file and reduce the live OST file size.
Why Outlook Has a 100 GB OST File Limit
The OST file is a local cache of your Exchange or Microsoft 365 mailbox. Outlook and the Windows Search indexer work together to provide fast search and offline access. The database engine that manages OST files, Extensible Storage Engine (ESE), has a practical maximum database size of 100 GB. Beyond that limit, the engine experiences fragmentation, write errors, and corruption.
Microsoft does not officially support OST files larger than 100 GB for any version of Outlook connected to Exchange Online or Exchange Server. If your OST file exceeds this limit, Outlook may show the error “Cannot start Microsoft Outlook. Cannot open the Outlook window. The set of folders cannot be opened.” Or synchronization may stop with the message “An unexpected error occurred. Your mailbox may be temporarily unavailable.”
The only reliable long-term fix is to reduce the size of the OST file by moving older items out of the live mailbox and into a separate archive file. This process does not delete any data. It moves messages, calendar entries, and tasks to a PST file that you can open in Outlook at any time.
Steps to Create an Archive PST and Move Older Mail
Before you start, close Outlook and make sure you have enough free disk space. The archive PST file will be stored on your local drive or a network location. You need at least 20 GB free space for a typical archive operation.
Step 1: Check Your Current OST File Size
- Open Outlook
Go to File > Account Settings > Account Settings. - Select the Data Files tab
You see a list of all data files. The OST file for your Exchange account is shown with the type “Offline Outlook Data File.” - Note the file location
Click the OST file row, then click Open File Location. Windows File Explorer opens to the folder containing your OST file. Right-click the file and select Properties. The Size value on the General tab shows the current file size. If it is near or above 100 GB, proceed to the next steps.
Step 2: Create a New Archive PST File
- Open Outlook
Go to File > Account Settings > Account Settings. - Click the Data Files tab
Click Add. - Choose the file type
In the Create or Open Outlook Data File dialog, select Outlook Data File (.pst). Click OK. - Name and save the file
Type a descriptive name like “Archive2024.pst” or “OldMailArchive.pst.” Choose a folder with enough free space. Click OK. - Set a password (optional)
You can add a password to the PST file. If you do, remember the password. Without it, you cannot open the archive later. Click OK.
The new PST file appears in the Navigation Pane under your mailbox folders. You can rename the archive root folder by right-clicking it and selecting Properties.
Step 3: Move or Copy Items to the Archive PST
- Open the Cleanup Tools menu
In Outlook, go to File > Info > Cleanup Tools > Archive. - Select the folder to archive
In the Archive dialog, choose Archive this folder and all subfolders. Select your mailbox root folder (usually your email address). - Set a date cutoff
Under Archive items older than, choose a date. For example, select 6 months or 1 year. Items older than that date will be moved to the archive PST. - Choose the archive file
Under Archive file, click Browse and select the PST file you created in Step 2. Make sure the option “Include items with ‘Do not AutoArchive’ checked” is not selected unless you want to move those items too. - Click OK
Outlook moves the items to the archive PST. The process may take several minutes depending on the number of items.
After archiving, the OST file size decreases. To verify, close Outlook, reopen it, and repeat Step 1 to check the file size again.
Step 4: Manual Drag-and-Drop for Selective Archiving
If you want to move only specific folders or individual messages, use drag-and-drop.
- Open the archive PST
In the Navigation Pane, expand the archive PST file you created. - Create subfolders (optional)
Right-click the archive root folder, select New Folder, and name it, for example, “2023 Emails” or “Projects.” - Drag items or folders
Select a folder from your live mailbox. Hold the Ctrl key and drag it to the archive PST folder. This copies the items. To move instead of copy, drag without the Ctrl key. Outlook moves the items and removes them from the live mailbox.
If Outlook Still Has Issues After Archiving
OST file size did not decrease
The OST file may not shrink immediately because Outlook reserves space. Close Outlook and run the Mailbox Cleanup tool. Go to File > Info > Cleanup Tools > Mailbox Cleanup. Click AutoArchive to force a full cleanup. If the file is still large, compact the OST file by going to File > Account Settings > Account Settings > Data Files, select the OST file, click Settings, then click Compact Now. This process can take up to 30 minutes.
Outlook shows “Cannot start Microsoft Outlook” after archiving
If the OST file was corrupted before archiving, the archive operation may fail. Delete the OST file and let Outlook rebuild it. Close Outlook. Open File Explorer and navigate to the OST file location from Step 1. Right-click the OST file and delete it. Restart Outlook. Outlook downloads a new OST file from the server. The archive PST file remains intact and contains your old items.
Archived items are missing from the PST file
Check that you selected the correct PST file during the archive operation. Open the archive PST by going to File > Open & Export > Open Outlook Data File. Browse to the PST file and click OK. If the items are not there, run the archive process again with a different date cutoff. If the PST file is corrupted, use the Inbox Repair Tool (scanpst.exe) located in the Outlook program folder. Run the tool on the PST file and follow the prompts.
AutoArchive vs Manual Archive vs Online Archive
| Item | AutoArchive | Manual Archive | Online Archive (Exchange Online) |
|---|---|---|---|
| Trigger | Runs automatically on a schedule | User initiates from File > Info > Cleanup Tools | Admin enables per mailbox in Exchange admin center |
| Destination | Local PST file you specify | Local PST file you choose each time | Cloud-based archive mailbox in Exchange Online |
| Access | Open PST file in Outlook | Open PST file in Outlook | Visible in Outlook Navigation Pane automatically |
| Effect on OST size | Reduces OST file size | Reduces OST file size | Reduces OST file size because items are moved out of primary mailbox |
| Limitations | PST file can also reach large size | PST file can also reach large size | Requires Exchange Online Plan 2 or Exchange Online Archiving add-on |
After moving mail to an archive PST, you can keep the file open in Outlook for quick access. To prevent future growth, set a recurring reminder to archive every three months. If your organization uses Exchange Online, ask your IT admin to enable the online archive feature, which moves items to a separate cloud mailbox without creating a local PST file.