A very large Outlook Offline Data File (.ost) can consume significant disk space and may contribute to slow Outlook performance, pauses, sluggish searches, synchronization problems, or difficulty moving and managing messages.

This is especially common with Microsoft 365 and Exchange mailboxes that contain years of email, large attachments, shared mailboxes, or folders configured for offline synchronization.

The good news is that an OST file is normally a local synchronized copy of mailbox information stored on the mail server. Therefore, you can often reduce its size without deleting important server-side email.

However, the correct method matters. Simply deleting random Outlook files is not a good cleanup strategy.

Quick Answer

If your Outlook OST file has become too large, try these methods in this order:

1. Check the actual OST file size.
2. Check which mailbox folders contain the most data.
3. Delete unnecessary emails and large attachments.
4. Empty Deleted Items.
5. Clean unnecessary mailbox content.
6. Compact the OST file.
7. Reduce Mail to keep offline from All to 12 months, 6 months, 3 months, or another suitable period.
8. Review shared-folder caching if shared mailboxes are making the OST unusually large.
9. Rebuild the OST file if it remains abnormally large or appears damaged.
10. Avoid increasing Outlook's OST/PST size limit unless there is a specific administrative requirement.

For many Microsoft 365 and Exchange users, reducing Mail to keep offline is one of the most effective long-term solutions because older messages remain on the server but are no longer all stored in the local OST cache.

What Is an Outlook OST File?

OST stands for Offline Outlook Data File.

Classic Outlook for Windows can create an .ost file when working with accounts such as:

• Microsoft 365
• Microsoft Exchange
• Outlook.com
• certain IMAP configurations

For Exchange and Microsoft 365 accounts using Cached Exchange Mode, Outlook stores a synchronized local copy of mailbox information so that users can access their mail quickly and continue working when the connection to the mail server is slow or temporarily unavailable.

A common OST location is:

C:\Users\<Username>\AppData\Local\Microsoft\Outlook\

For example:

C:\Users\Balvinder\AppData\Local\Microsoft\Outlook\

The AppData folder is hidden by default.

A quick way to open the folder is:

1. Press Windows + R.
2. Enter:

%LOCALAPPDATA%\Microsoft\Outlook

3. Press Enter.

You can then check the size of any .ost files present.

OST vs PST: Understand the Difference Before Deleting Anything

Feature	OST	PST
Meaning	Offline Outlook Data File	Outlook Data File
Main purpose	Local synchronized mailbox cache	Local Outlook data storage/archive
Common with Microsoft 365/Exchange	Yes	Optional
Common with POP accounts	No	Yes
Normally synchronized with server	Yes	No
Can usually be recreated from server	Yes	Not automatically
Should you casually delete it?	No	Absolutely not

This difference is extremely important.

An OST file for a healthy server-based mailbox can normally be recreated by synchronizing the mailbox again. A PST file, however, may contain the only copy of locally stored messages.

Never treat OST and PST files as interchangeable when performing cleanup or deletion.

Why Does an OST File Become So Large?

An OST file can grow for several reasons.

1. Large Microsoft 365 or Exchange Mailbox

If the mailbox contains many years of email and Outlook is configured to synchronize all of it, the local OST can become very large.

2. Large Email Attachments

Messages containing:

• PDF files
• ZIP archives
• videos
• photographs
• presentations
• database backups
• large Excel workbooks

can consume significant mailbox and OST storage.

3. Mail to Keep Offline Is Set to All

In classic Outlook, Cached Exchange Mode can synchronize a specified period of email.

If Mail to keep offline is set to:

All

Outlook may cache years of mailbox data locally.

4. Deleted Items Have Not Been Emptied

Deleting an email normally moves it into Deleted Items.

Until Deleted Items is emptied, those messages continue consuming mailbox storage.

5. Shared Mailboxes and Shared Folders

Shared folders can significantly increase OST size when Outlook is configured to download their contents.

This is particularly noticeable for users who have access to multiple large shared mailboxes.

6. Public Folder Caching

Organizations using Exchange public folders can also accumulate substantial locally cached data.

7. OST File Does Not Immediately Shrink After Deleting Email

This surprises many Outlook users.

Deleting several gigabytes of email does not necessarily reduce the physical OST file by the same amount immediately.

The deleted information may create unused space inside the data file. Outlook can reuse that space later, and the data file may be compacted automatically or manually.

Is a Large OST File Really a Problem?

Not necessarily.

A 10 GB OST file is not automatically damaged simply because it is 10 GB.

However, Microsoft documents that large Outlook data files can cause application pauses, with the impact depending on data-file size and computer hardware.

Large files may contribute to:

• slower Outlook startup
• delays opening folders
• slower searches
• pauses when moving messages
• slower deletion operations
• synchronization delays
• higher disk activity
• greater local storage usage

The number of items in individual folders can also affect Outlook performance, so file size alone should not be used as the only performance indicator.

Method 1: Use Outlook Mailbox Cleanup

Before changing advanced Outlook settings, identify what is consuming mailbox space.

In classic Outlook:

1. Open Outlook.
2. Select File.
3. Select Tools.
4. Select Mailbox Cleanup.

Depending on your Outlook configuration, Mailbox Cleanup can help you:

• view mailbox size
• find messages older than a specified number of days
• find messages larger than a specified size
• check Deleted Items size
• empty Deleted Items
• check conflict data

This is one of the safest places to start.

Method 2: Find and Remove Large Emails

Large attachments are often responsible for excessive mailbox growth.

In classic Outlook, you can use Outlook Search to locate large messages.

For example:

messagesize:>5 MB

This searches for messages larger than approximately 5 MB.

You can adjust the value according to your requirements.

Before deleting anything, determine whether the message or attachment is still required.

For business email, consider organizational retention requirements before permanently deleting records.

Method 3: Empty Deleted Items

After deleting unnecessary email:

1. Locate Deleted Items.
2. Right-click the folder.
3. Select Empty Folder.

Alternatively, use Outlook's Mailbox Cleanup tools.

Important

Emptying Deleted Items permanently removes those messages from the normal mailbox folder structure. Confirm that the messages are no longer required before doing this.

Also remember that organizational retention or recoverable-item policies may affect when data is actually removed from the server.

Method 4: Compact the Outlook OST File

After deleting a significant amount of data, the physical OST file may remain large because unused space can remain inside it.

Microsoft provides a built-in compaction function in classic Outlook.

How to Compact an OST File

1. Open Outlook.
2. Select:

File → Account Settings → Account Settings

3. Open the Data Files tab.
4. Select the relevant data file.
5. Click Settings.

For Exchange or Outlook.com configurations, you may need to select:

Advanced → Outlook Data File Settings

6. Select:

Compact Now

7. Allow the process to complete.

Depending on the size of the file and computer performance, compaction may take some time.

Why Compact Now May Not Dramatically Reduce the File

Compaction only removes unused internal space. It does not magically remove email that is still being synchronized.

Therefore, if your mailbox contains 30 GB of data and Outlook is configured to cache all 30 GB, compaction cannot reduce it to a few gigabytes.

You must first reduce the data being stored locally.

Method 5: Reduce “Mail to Keep Offline”

For Microsoft 365 and Exchange users, this is often the most useful solution.

Instead of keeping your entire mailbox locally, configure Outlook to cache only recent email.

In classic Outlook:

1. Open File.
2. Select:

Account Settings → Account Settings

3. Select your Microsoft 365 or Exchange account.
4. Click Change.
5. Locate:

Mail to keep offline

6. Change the slider from All to an appropriate period, such as:

• 12 months
• 6 months
• 3 months
• 1 month

Available choices can vary by Outlook version and configuration.

7. Complete the wizard if prompted.
8. Restart Outlook.
9. Allow Outlook time to synchronize.

For example, if you select 3 months, Outlook keeps recent mailbox data locally while older messages remain available from the server rather than all being retained in the local OST cache.

Recommended Setting

There is no universal best setting.

User Type	Possible Starting Point
Normal office user	12 months
Large mailbox	6 months
Very large mailbox	3 months
RDS/VDI environment with limited storage	1 month or organizationally appropriate minimum
Frequent offline user	Longer cache period may be preferable

These are practical recommendations rather than Microsoft requirements.

The correct choice depends on:

• mailbox size
• available SSD/storage
• internet reliability
• how frequently old email is accessed
• whether users work offline
• organizational policy

Important Note for RDS and Virtual Desktop Environments

OST files can be especially important in:

• Remote Desktop Services
• Windows Server multi-user environments
• virtual desktops
• profile-container environments

If many users each have a large OST file, local or profile storage consumption can become substantial.

For example, 20 users each caching 20 GB could theoretically require hundreds of gigabytes of storage before accounting for other profile data.

Microsoft specifically notes that reducing the amount of synchronized mail can be useful in virtual desktop environments.

Administrators should therefore consider controlling Cached Exchange Mode and offline synchronization according to the architecture of their RDS/VDI environment.

Method 6: Check Shared Mailbox and Shared Folder Caching

A user's own mailbox may not be the only data stored in the OST.

If the user has access to:

• Accounts
• Sales
• Support
• HR
• Helpdesk
• other shared mailboxes

Outlook may cache shared-folder content locally.

This can cause the OST file to become unexpectedly large.

In classic Outlook, check:

File → Account Settings → Account Settings → select account → Change → More Settings → Advanced

Look for settings related to:

Download shared folders

The exact interface can vary by Outlook build and Exchange configuration.

Should Download Shared Folders Be Disabled?

Not automatically.

Disabling shared-folder caching can reduce OST size in suitable environments, but it changes how shared folders are accessed and can affect performance and offline availability.

Administrators should evaluate this according to:

• network performance
• number of shared mailboxes
• shared mailbox sizes
• offline-access requirements
• RDS/VDI architecture

Method 7: Archive Older Email to a PST File

If organizational policy permits local archives, older email can be moved into a separate Outlook Data File (.pst).

This can reduce the amount of information in the active mailbox and, after synchronization and compaction, may help reduce the OST.

However, PST archives introduce additional management requirements.

Important PST Warning

A PST is not simply a disposable cache like an OST generally is.

If important messages are moved from the server mailbox into a PST, that PST may become the only copy of those messages.

Therefore:

• maintain proper backups
• do not casually delete PST files
• avoid storing active PST files in unsupported locations
• follow your organization's retention and backup policies

For organizations using Microsoft 365 retention, online archives, or centrally managed compliance policies, server-side archiving may be preferable to manually created PST archives.

Method 8: Rebuild the OST File

If the OST remains unusually large, synchronization behaves incorrectly, or the local cache appears damaged, rebuilding the OST may help.

Before Rebuilding

First verify that:

• Outlook can connect to the mail server.
• Required email is visible on the server/webmail.
• Synchronization has completed.
• You understand whether any unsynchronized local changes exist.

Important Warning

Do not delete an OST simply because an article says that OST files are normally recreatable.

If Outlook contains unsynchronized changes, local-only information, or an unusual account configuration, deleting the cache could result in data loss.

Safe General Procedure to Rebuild an OST

1. Confirm that important mailbox data exists on the server.
2. Close Outlook completely.
3. Open:

%LOCALAPPDATA%\Microsoft\Outlook

4. Locate the appropriate .ost file.
5. Instead of immediately deleting it, rename it.

Example:

user@domain.com.ost

to:

user@domain.com.ost.old

6. Start Outlook.

For a properly configured server-based account, Outlook should create a new OST and synchronize the mailbox again.

Why Rename Instead of Delete?

Renaming provides a temporary fallback while troubleshooting.

After confirming that:

• Outlook is functioning correctly,
• synchronization has completed, and
• all required mailbox information is available,

the old file can be removed if it is no longer required.

Method 9: Create a New Outlook Profile

Sometimes the problem is not just the OST file. The Outlook profile itself may have configuration or synchronization problems.

You can test with a new Outlook profile.

1. Close Outlook.
2. Open Control Panel.
3. Search for:

Mail

4. Open Mail (Microsoft Outlook).
5. Select Show Profiles.
6. Select Add.
7. Create a new profile.
8. Configure the email account.
9. Test Outlook using the new profile.

If Outlook works correctly with a new profile, the previous Outlook profile may have been contributing to the problem.

Do not remove the old profile until you have verified that everything required is available in the new one.

What Is the Maximum OST File Size?

Microsoft documents a default maximum size of 50 GB for Unicode PST and OST files in Outlook 2010, 2013, and 2016, with a default warning threshold of approximately 47.5 GB.

These limits can be controlled through registry values such as:

MaxLargeFileSize
WarnLargeFileSize

For Office/Outlook installations using the 16.0 registry branch, relevant configuration can exist below locations such as:

HKEY_CURRENT_USER\Software\Microsoft\Office\16.0\Outlook\PST

or policy-controlled locations such as:

HKEY_CURRENT_USER\Software\Policies\Microsoft\Office\16.0\Outlook\PST

Despite the registry subkey being named PST, Microsoft documents these size-control settings as applying to both PST and OST files.

Should You Increase the OST Limit Above 50 GB?

Usually, reducing the amount of cached data is preferable to simply increasing the limit.

Microsoft notes that Unicode size limits can technically be configured beyond documented defaults but does not recommend doing so because performance can decrease.

Increasing the maximum file size does not solve the underlying causes of:

• excessive mailbox data
• unnecessary local caching
• shared mailbox caching
• huge attachments
• overloaded folders
• insufficient local disk space

For this reason, increasing the OST limit should generally be an administrative exception rather than the first troubleshooting step.

Common Mistakes to Avoid

Mistake 1: Deleting the OST Without Checking Synchronization

Always verify that required information has synchronized to the server first.

Mistake 2: Confusing OST with PST

An OST is normally a synchronized cache.

A PST may contain unique local information.

Never delete a PST using OST troubleshooting instructions.

Mistake 3: Deleting Email but Not Emptying Deleted Items

Messages in Deleted Items can continue consuming mailbox storage.

Mistake 4: Expecting the OST to Shrink Immediately

Deleting messages creates unused space, but the physical file does not necessarily shrink immediately.

Compaction may be required.

Mistake 5: Increasing the 50 GB Limit as the First Solution

This can postpone the problem instead of solving it.

Mistake 6: Ignoring Shared Mailboxes

A user with a moderate mailbox can still have a huge OST because of cached shared folders.

Mistake 7: Keeping “All” Email Offline Without Needing It

If a user has 10 years of email but normally works only with recent messages, caching the entire mailbox may provide little benefit while consuming significant disk space.

Recommended Troubleshooting Sequence

For most users, use this order:

Step 1: Check OST size.

Step 2: Check mailbox and folder sizes.

Step 3: Find unusually large messages.

Step 4: Delete unnecessary email.

Step 5: Empty Deleted Items.

Step 6: Compact the OST.

Step 7: Reduce Mail to keep offline if the mailbox is still too large.

Step 8: Check shared-folder caching.

Step 9: Allow Outlook to fully synchronize.

Step 10: Rebuild the OST if the local cache still appears abnormal.

Step 11: Test with a new Outlook profile if problems continue.

This approach addresses the cause before resorting to more disruptive troubleshooting.

New Outlook vs Classic Outlook

This distinction is important.

Many of the menu paths described in this article—particularly:

• Cached Exchange Mode
• Mail to keep offline
• Data Files
• Compact Now
• traditional OST management

refer to classic Outlook for Windows.

Microsoft states that Cached Exchange Mode as configured in classic Outlook is not available in the same way in new Outlook for Windows.

Therefore, if instructions on your computer do not match the screenshots or menu paths described here, first determine whether you are using:

Classic Outlook

or

New Outlook for Windows

Do not assume instructions written for classic Outlook apply identically to new Outlook.

Frequently Asked Questions

Why is my Outlook OST file so large?

Common reasons include a large mailbox, many years of synchronized email, large attachments, Cached Exchange Mode configured to keep all email offline, and cached shared mailboxes or folders.

Can I safely delete an OST file?

For a healthy Microsoft 365 or Exchange account, an OST is normally a synchronized local copy that Outlook can recreate from the server. However, you should not delete it blindly. Verify server-side data and synchronization first. Renaming the file is generally safer during troubleshooting.

Will deleting emails reduce OST size immediately?

Not necessarily. Outlook can retain unused space inside the OST for future use. Use Compact Now after removing significant amounts of data if you need to reduce the physical file size.

Where is the OST file stored?

A common location is:

C:\Users\<Username>\AppData\Local\Microsoft\Outlook\

You can quickly open it with:

%LOCALAPPDATA%\Microsoft\Outlook

How do I compact an OST file?

In classic Outlook, use:

File → Account Settings → Account Settings → Data Files → Settings

For Exchange or Outlook.com accounts, you may then need:

Advanced → Outlook Data File Settings → Compact Now

The exact interface can vary by Outlook version.

What does “Mail to keep offline” do?

It determines how much mailbox history Outlook stores locally for offline access in Cached Exchange Mode.

For example, selecting 3 months keeps recent mail locally while older server-side mail remains available when accessed through the connected mailbox.

Does reducing Mail to keep offline delete old emails?

No. For a server-based Microsoft 365 or Exchange mailbox, reducing the synchronization period changes how much mail is stored in the local offline cache. Older server-side messages are not simply deleted because you reduce the slider.

Is a 20 GB OST file too large?

Not automatically. Whether it causes performance problems depends on computer hardware, storage performance, mailbox structure, folder item counts, and Outlook configuration. However, reducing unnecessary cached data can improve storage usage and sometimes Outlook responsiveness.

What happens when an OST reaches 50 GB?

For Outlook versions using the documented default 50 GB Unicode data-file limit, reaching the configured limit can prevent the file from accepting additional data and may cause synchronization or item-management problems.

Can I increase the OST limit beyond 50 GB?

Registry settings can change Outlook data-file limits, but increasing the limit is generally not the preferred solution. Microsoft warns that increasing Unicode data-file limits beyond documented values can reduce performance.

Does Compact Now delete email?

No. Compaction is intended to reclaim unused space within the Outlook data file. It does not serve as a command to delete valid mailbox messages.

Why is the OST still large after deleting thousands of emails?

The physical file can contain reusable unused space. Empty Deleted Items, allow synchronization to finish, and then compact the data file. Also check whether the mailbox or shared folders are still synchronizing large amounts of data.

Can shared mailboxes increase OST size?

Yes. When shared folders are downloaded and cached locally, their content can contribute substantially to the OST file.

Should I disable Cached Exchange Mode?

Usually not merely to save disk space. Microsoft recommends Cached Exchange Mode for Microsoft 365 mailboxes in classic Outlook. Reducing the amount of mail stored offline is often a better solution.

Do I need to back up an OST file?

Normally, an OST is a synchronized copy of information stored on the mail server and Microsoft states that it does not need to be backed up like a PST. However, before rebuilding an OST during troubleshooting, verify that important information has successfully synchronized to the server.

Can I use ScanPST to reduce OST size?

ScanPST is primarily a data-file repair utility, not a routine method for reducing OST size. For an oversized but otherwise healthy OST, first use mailbox cleanup, synchronization controls, compaction, and—where appropriate—OST recreation.

Best Practices for Preventing Oversized OST Files

For long-term Outlook performance:

• avoid synchronizing years of email locally unless required
• periodically remove unnecessary large messages
• empty Deleted Items when appropriate
• monitor mailbox growth
• review shared mailbox caching
• use suitable Microsoft 365/Exchange retention and archive solutions
• keep sufficient free space on the Windows system drive
• use SSD storage where possible
• keep Outlook and Microsoft 365 Apps updated
• periodically review RDS/VDI user-profile storage
• avoid unnecessary registry modifications to increase data-file limits

Conclusion

A large Microsoft Outlook OST file does not automatically mean that Outlook is corrupted. In many cases, the file is large simply because Outlook is caching a large Microsoft 365 or Exchange mailbox.

The safest approach is to determine why the OST is large before attempting to remove it.

Start by cleaning unnecessary mailbox data, emptying Deleted Items, and compacting the data file. For large Microsoft 365 or Exchange mailboxes, reducing Mail to keep offline can significantly reduce the amount of information Outlook needs to store locally. Shared-mailbox caching should also be checked when the OST is unexpectedly large.

Rebuilding the OST is useful when the local cache appears damaged or abnormal, but it should be done only after confirming that important mailbox information has synchronized successfully with the server.

Most importantly, do not confuse an OST with a PST: an OST is generally a synchronized cache, while a PST may contain the only copy of important locally stored email.

 

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