If you’ve landed here because you’re seeing the How to Fix Outlook Error “The File [.ost] Has Reached Its Maximum Size” message, you’re not alone. I ran into this issue after years of using the same Outlook profile without thinking much about mailbox size. One morning Outlook became slow, stopped syncing emails, and eventually displayed the dreaded message saying the OST file had reached its maximum size. At first, I thought my email account was broken, but the real problem was much simpler.

Fortunately, this error is usually fixable without reinstalling Outlook or losing your emails. In this guide, you’ll learn exactly why it happens, how to solve it, and how to stop it from happening again.

What Does “The File [.ost] Has Reached Its Maximum Size” Mean?

An OST (Offline Storage Table) file is Outlook’s local copy of your mailbox. It lets you access emails, calendars, contacts, and other data even when you’re offline.

As your mailbox grows over months or years, the OST file becomes larger. Once it reaches Outlook’s configured storage limit, Outlook may stop syncing new emails or display the following error:

The file [.ost] has reached its maximum size.

When this happens, Outlook can become slow, freeze, or refuse to receive new messages.

Why Does This Error Happen?

Several factors can cause your OST file to become too large.

Common causes include:

• Large email archives
• Thousands of emails with attachments
• Multiple shared mailboxes
• Cached Exchange Mode storing too much data
• Old calendar items
• Large Deleted Items folder
• Outlook running for many years without cleanup

The problem is especially common in Microsoft 365 and Exchange accounts where Outlook continuously caches mailbox data.

Step 1: Check Your Mailbox Size

The first thing you should do is determine whether your mailbox is actually too large.

Open Outlook.

Go to:

File → Tools → Mailbox Cleanup

Here you can:

• View your mailbox size
• Find large emails
• Locate old messages
• Empty Deleted Items

If your mailbox is several gigabytes in size, reducing it is usually the fastest solution.

Step 2: Empty the Deleted Items Folder

Many people never permanently delete emails.

Even after deleting messages, Outlook keeps them inside the Deleted Items folder until you remove them permanently.

To clean it:

1. Open Deleted Items.
2. Select everything.
3. Delete permanently.
4. Empty the folder.

This immediately frees space inside the OST file.

Step 3: Remove Large Attachments

Attachments usually consume most of your mailbox storage.

Search for messages larger than 10 MB.

You can:

• Save attachments to your computer
• Upload them to cloud storage
• Remove attachments from emails you no longer need

This dramatically reduces mailbox size.

Step 4: Archive Older Emails

If you rarely access emails older than one or two years, archive them.

To archive:

1. Open Outlook.
2. Select File.
3. Choose Tools.
4. Click Clean Up Old Items.
5. Select the folders to archive.
6. Choose an archive location.
7. Start the archive process.

Archiving removes older emails from your active mailbox while keeping them accessible later.

Step 5: Compact the OST File

Deleting emails doesn’t immediately shrink the OST file.

You need to compact it.

Follow these steps:

1. Close unnecessary Outlook windows.
2. Go to:

File → Account Settings → Account Settings

3. Open the Data Files tab.
4. Select your OST file.
5. Click Settings.
6. Choose Compact Now.

Depending on the file size, this may take several minutes.

Step 6: Reduce Cached Email Duration

If you’re using Microsoft 365 or Exchange, Outlook doesn’t need to cache every email from the past decade.

Instead:

1. Open Account Settings.
2. Select your Exchange account.
3. Click Change.
4. Find the slider labeled:

Download email for the past

Instead of:

• All

Choose:

• 12 months
• 6 months
• 3 months

Outlook will rebuild a much smaller OST file while older emails remain available online.

Step 7: Disable Unnecessary Shared Mailboxes

Shared mailboxes also increase OST size.

If Outlook downloads several shared inboxes, the OST file grows much faster.

Ask your administrator whether every shared mailbox is necessary.

Removing unused shared folders can significantly reduce storage.

Step 8: Create a New OST File

Sometimes the OST file becomes corrupted in addition to being oversized.

You can safely rebuild it.

Steps:

1. Close Outlook.
2. Open Control Panel.
3. Go to Mail.
4. Open Data Files.
5. Note the OST location.
6. Close Outlook.
7. Rename the OST file.
8. Start Outlook again.

Outlook automatically creates a fresh OST file and downloads mailbox contents again.

Step 9: Repair Outlook Profile

If rebuilding the OST doesn’t solve the issue, repair your Outlook profile.

Go to:

Control Panel → Mail → Email Accounts

Select your account.

Choose:

Repair

Follow the wizard until it finishes.

Then restart Outlook.

Step 10: Check Mailbox Quotas

Sometimes the issue isn’t the OST file itself.

Your Exchange administrator may have configured mailbox storage limits.

If your mailbox quota is full:

• Delete unnecessary emails
• Empty Deleted Items
• Empty Junk Email
• Archive older content
• Contact your administrator if additional storage is needed

How Large Can an OST File Become?

The limit depends on your Outlook version.

Typical limits include:

Administrators can increase this limit through the Windows Registry, but Microsoft generally recommends reducing mailbox size instead.

Can You Increase the OST Size Limit?

Technically, yes.

Administrators can modify registry values such as:

• MaxLargeFileSize
• WarnLargeFileSize

However, increasing the limit is usually only a temporary fix.

A massive OST file often causes:

• Slow Outlook startup
• Search problems
• Synchronization failures
• Higher corruption risk

Keeping the mailbox smaller is a better long-term solution.

Tips to Prevent the Error

You can avoid this problem by following a few simple habits.

Clean your mailbox monthly

Delete unnecessary emails regularly.

Empty Deleted Items

Don’t let deleted emails accumulate.

Remove large attachments

Save files elsewhere whenever possible.

Archive older emails

Keep only recent messages in your active mailbox.

Reduce Cached Exchange Mode

Download only the emails you actually need offline.

Monitor mailbox size

Check mailbox usage every few months.

Common Mistakes to Avoid

Many users accidentally make the situation worse.

Avoid these mistakes:

• Deleting the OST file while Outlook is still open
• Editing registry values without understanding them
• Ignoring mailbox warnings
• Keeping thousands of unnecessary attachments
• Creating multiple Outlook profiles without cleanup

When Should You Contact IT Support?

Consider contacting your IT department if:

• Outlook won’t open after rebuilding the OST.
• Synchronization never finishes.
• Your mailbox quota is managed by your organization.
• Outlook crashes repeatedly.
• The OST file becomes corrupted multiple times.

Business environments often have server-side policies that require administrator assistance.

Frequently Asked Questions

Why did my OST file suddenly become too large?

It usually grows gradually as Outlook caches emails, attachments, calendars, contacts, and shared mailboxes. Many users don’t notice until the file reaches Outlook’s configured limit.

Will deleting the OST file delete my emails?

No. For Exchange and Microsoft 365 accounts, the OST file is only a local cache. Outlook downloads the data again after creating a new OST file.

Is it safe to compact an OST file?

Yes. Compacting removes unused space and is one of Microsoft’s recommended maintenance tasks.

Can Outlook automatically reduce OST size?

No. Outlook does not automatically shrink the file after emails are deleted. You need to compact it manually or rebuild the OST file.

Should I increase the registry limit instead?

Only if you fully understand the consequences and your administrator recommends it. Reducing mailbox size is generally a safer and more reliable solution.

Does archiving delete my emails?

No. Archiving simply moves older emails into a separate Outlook archive file, allowing you to access them later while reducing the size of your active mailbox.

Final Thoughts

The How to Fix Outlook Error “The File [.ost] Has Reached Its Maximum Size” message usually looks more serious than it really is. In most cases, cleaning your mailbox, removing unnecessary attachments, compacting the OST file, or rebuilding it completely will restore Outlook to normal operation.

The best long-term strategy is regular mailbox maintenance. Spending just a few minutes each month deleting old messages and archiving emails can prevent this error from returning and keep Outlook running smoothly for years.

Editor’s Opinion

I think lots of people ignore Outlook until it stop working. I did the same before. Most times the fix is not hard, just clean old emails and big files. Dont wait too long because a huge mailbox only makes everything slower. Small cleanup every month really save time later.