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."

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

Product / System Overview

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

Affected Components:

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

Outlook Data File Size Limits

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

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

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

Troubleshooting Workflow

Step 1 – Confirm File Type & Size

Locate Data File

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

Check:

✔ File type (PST / OST)
✔ Current size

Manual Verification

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

Right-click → Properties → Size

Step 2 – Identify Limit Condition

If file size is near:

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

Proceed with remediation.

Resolution Methods

Method 1 – Reduce Mailbox Size (Immediate Fix)

Recommended Cleanup Areas

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

Permanent Deletion

Use:

SHIFT + DELETE

Important:

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

Mailbox Cleanup Tool

Outlook →

File → Tools → Mailbox Cleanup

Useful for:

✔ Finding large items
✔ Emptying folders

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

Method 3 – Increase Outlook File Size Limit (Advanced)

Use when cleanup is insufficient.

Registry Configuration

Open Registry Editor

Win + R → regedit

Navigate to Version Path

Example:

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

Create DWORD Values

Create:

MaxLargeFileSize
WarnLargeFileSize

Example – Set Limit to 100 GB

MaxLargeFileSize  = 102400
WarnLargeFileSize = 95000

✔ Select Decimal
✔ Value in MB

Size Reference Table

⚠ Warning: Very large files degrade Outlook performance.

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

✔ Enable AutoArchive
✔ Create Secondary PST
✔ Move older mail

Manual Archive

Outlook →

File → Tools → Clean Up Old Items

Method 5 – OST File Rebuild (Exchange / IMAP)

Useful when OST corruption contributes.

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

Common Root Causes

Common Errors & Fixes

Security Considerations

Increasing file size limits introduces risks:

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

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

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.