Outlook Error 0x8404060C – Message Store Reached Maximum Size (Increase Outlook Email PST Size thru Windows Registry)

This article provides technical guidance for diagnosing and resolving Microsoft Outlook Send/Receive Error 0x8404060C, typically accompanied by:

Error 0x8404060C
"The message store has reached its maximum size. To reduce the amount of data in this message store, select some items that you no longer need, and permanently (SHIFT + DEL) delete them."

The guidance applies to IT professionals, system administrators, and support engineers managing Outlook environments using PST/OST files.

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Product / System Overview

Affected Product:
Microsoft Outlook (2010, 2013, 2016, 2019, Microsoft 365)

Affected Components:

Component	Description
PST	Personal Storage Table (local data file)
OST	Offline Storage Table (cached Exchange / IMAP data)

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Technical Explanation

What Triggers Error 0x8404060C

Outlook displays this error when the associated data file:

✔ Reaches the configured maximum file size limit
✔ Cannot expand further
✔ Blocks Send/Receive operations

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Outlook Data File Size Limits

File Type	Default Limit (Modern Outlook)
Unicode PST/OST	~50 GB
ANSI PST (Legacy)	~2 GB

Modern Outlook versions use Unicode format, but limits still apply.

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Underlying Mechanism

Outlook enforces size thresholds via registry-controlled parameters:

• MaxLargeFileSize → Absolute limit
• WarnLargeFileSize → Warning threshold

Once exceeded:

✔ Mail flow stops
✔ Sync errors occur
✔ Performance degrades

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Typical Use Cases / Environments

This issue commonly appears in:

✔ High-mail-volume users
✔ Shared mailbox caching
✔ IMAP accounts with large archives
✔ Systems with disabled archiving policies
✔ Long-running Outlook profiles

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Troubleshooting Workflow

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Step 1 – Confirm File Type & Size

Locate Data File

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

Check:

✔ File type (PST / OST)
✔ Current size

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Manual Verification

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

Right-click → Properties → Size

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Step 2 – Identify Limit Condition

If file size is near:

✔ 49–50 GB → Default Unicode limit
✔ ~2 GB → Legacy ANSI file

Proceed with remediation.

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Resolution Methods

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Method 1 – Reduce Mailbox Size (Immediate Fix)

Recommended Cleanup Areas

✔ Deleted Items
✔ Junk Email
✔ Sent Items
✔ Large Attachments

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Permanent Deletion

Use:

SHIFT + DELETE

Important:

✔ Normal delete = moves to Deleted Items
✔ SHIFT+DEL = actual removal

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Mailbox Cleanup Tool

Outlook →

File → Tools → Mailbox Cleanup

Useful for:

✔ Finding large items
✔ Emptying folders

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Method 2 – Compact Outlook Data File

Reduces internal white space.

Outlook →

File → Account Settings → Data Files → Settings → Compact Now

✔ Safe operation
✔ May take time on large files

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Method 3 – Increase Outlook File Size Limit (Advanced)

Use when cleanup is insufficient.

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Registry Configuration

Open Registry Editor

Win + R → regedit

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Navigate to Version Path

Example:

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

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Create DWORD Values

Create:

MaxLargeFileSize
WarnLargeFileSize

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Example – Set Limit to 100 GB

MaxLargeFileSize  = 102400
WarnLargeFileSize = 95000

✔ Select Decimal
✔ Value in MB

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Size Reference Table

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⚠ Warning: Very large files degrade Outlook performance.

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Method 4 – Archive Strategy (Best Long-Term Fix)

✔ Enable AutoArchive
✔ Create Secondary PST
✔ Move older mail

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Manual Archive

Outlook →

File → Tools → Clean Up Old Items

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Method 5 – OST File Rebuild (Exchange / IMAP)

Useful when OST corruption contributes.

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Steps

1. Close Outlook

2. Navigate:

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

3. Rename OST file:

mailbox.ost → mailbox.old

4. Restart Outlook

✔ Outlook rebuilds cache

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Common Root Causes

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Common Errors & Fixes

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Security Considerations

Increasing file size limits introduces risks:

⚠ Larger PST = Higher corruption probability
⚠ Longer backup times
⚠ Slower search/indexing
⚠ Increased disk I/O

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Best Practices & Recommendations

✔ Keep PST/OST under 20–30 GB for stability
✔ Implement archiving policies
✔ Avoid storing large attachments locally
✔ Use Exchange retention rules
✔ Periodically compact data files
✔ Disable shared mailbox caching if unnecessary

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Conclusion

Error 0x8404060C is fundamentally a storage limit condition, not a connectivity failure.

Most stable resolution:

✔ Reduce mailbox size
✔ Archive aggressively
✔ Avoid excessive registry expansion

Registry tuning should be considered secondary, not primary.

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