Are you facing the frustrating Outlook error, “Cannot move the items. The message store has reached its maximum size?” You’re not alone. This is a common issue for users with large IMAP or Exchange mailboxes, and it effectively stops you from sending, receiving, or organizing your email.

your can follow along Video Tutorial at the end of this post.

This error occurs because modern versions of Outlook (2016, 2019, and Microsoft 365) impose a default size limit of 50 GB on the local data file (.ost) it creates to cache your mailbox. While other clients like Thunderbird don’t have this specific limitation, if you want to continue using Outlook, you need a permanent fix.

This guide will walk you through the definitive solution: editing the Windows Registry to increase Outlook’s maximum data file size limit.

The Primary Solution: Increase the Data File Size Limit via Registry Editor

This procedure directly tells Outlook to allow a larger data file, solving the root cause of the problem by increasing the limit from 50 GB to 100 GB.

⚠️ Important Disclaimer: This guide involves editing the Windows Registry. Incorrect changes can cause serious system instability. Please follow these steps exactly. It’s always a good practice to back up your registry before proceeding (in the Registry Editor, go to File > Export).

Step 1: Close Outlook and Open Registry Editor

  1. Ensure Microsoft Outlook is completely closed.

  2. Press the Windows Key + R on your keyboard to open the “Run” dialog.

  3. Type regedit and press Enter.

  4. A User Account Control (UAC) prompt will appear. Click Yes to continue.

Step 2: Navigate to the Correct Outlook Path In the Registry Editor window, use the folder tree on the left to navigate to the following path. The 16.0 key corresponds to Outlook 2016, 2019, and Microsoft 365.

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

  • For Outlook 2013: Use 15.0 instead of 16.0.

  • For Outlook 2010: Use 14.0 instead of 16.0.

(Note: Even though you are fixing an .ost file, the size limits for both .ost and .pst files are controlled under this PST key in the registry.)

Step 3: Create the Registry Values Once you have the PST folder selected, look at the right-hand pane. We need to create two new values here.

  1. Right-click on the empty white space in the right pane.

  2. Select New > DWORD (32-bit) Value.

  3. Name this new value exactly MaxFileSize.

  4. Right-click again in the white space.

  5. Select New > DWORD (32-bit) Value.

  6. Name this second value exactly WarnFileSize.

Step 4: Set the New Size Limits Now we’ll configure these values to set a new 100 GB limit.

  1. Double-click on your new MaxFileSize entry.

  2. In the edit window, click the Decimal radio button.

  3. In the “Value data” box, enter 102400 (this is 100 GB expressed in megabytes).

  4. Click OK.

  5. Next, double-click on your new WarnFileSize entry.

  6. Click the Decimal radio button.

  7. In the “Value data” box, enter 97280 (this sets the warning threshold to 95 GB, providing a 5% buffer).

  8. Click OK.

Step 5: Restart Your Computer Close the Registry Editor and restart your computer. This ensures the changes are fully applied. Once you log back in, open Outlook. The error will be gone, and Outlook will begin synchronizing your mail again.

What if the Fix Doesn’t Work? Troubleshooting Steps

Sometimes, even after following the steps perfectly, the error persists. This usually happens for one of two reasons. Here’s how to solve them.

1. Check for an Overriding Group Policy (Common in Work Environments)

An IT department can enforce settings that override your manual changes.

  • In regedit, navigate to the policy path: HKEY_CURRENT_USER\Software\Policies\Microsoft\Office\16.0\Outlook\PST.

  • If you see MaxFileSize or WarnFileSize values here, they are taking precedence. You would need administrator rights to change them, or contact your IT department.

  • If this path does not exist, then this is not the issue.

2. Recreate the Outlook Data File (.ost) – The Most Likely Solution

If the registry settings are correct and there is no policy override, the existing .ost file itself may be “stuck” at its old size limit. The solution is to force Outlook to create a fresh one.

This is safe for IMAP accounts as all your email is stored on the server.

  1. Close Outlook Completely. Use Task Manager (Ctrl+Shift+Esc) to ensure outlook.exe is not running.

  2. Open File Explorer.

  3. In the address bar, paste the following and press Enter: %localappdata%\Microsoft\Outlook

  4. You will see your data file (e.g., [email protected]).

  5. Rename the file. Right-click it and rename it to add .old to the end (e.g., [email protected]).

  6. Restart Outlook.

Outlook will not find its data file and will start re-downloading a fresh copy of your mailbox from the server. This new .ost file will be created with your new 100 GB size limit in effect. This process can take some time depending on your mailbox size, but it is the most reliable way to resolve a persistent error.

Alternative Strategies & Performance Tips

While increasing the limit is the best fix, Outlook can sometimes become slow with files over 75 GB. If you experience performance issues, consider these alternatives.

  1. Reduce the Local Cache (The Compromise)

    • What it does: Instructs Outlook to only keep recent emails (e.g., the last 12 months) in the local .ost file, keeping it small.

    • How: Go to File > Account Settings > Account Settings. Double-click your IMAP account and adjust the “Mail to keep offline” slider.

    • The Trade-off: Older emails are not stored locally. Searching for them is much slower as Outlook must query the server live, and results may be incomplete.

  2. Archive Older Emails to a Local PST File (The Hybrid Approach)

    • What it does: Moves emails older than a specified date from the server into a separate .pst file stored locally on your computer.

    • How: Go to File > Info > Tools > Clean Up Old Items (Archive).

    • The Trade-off: Archived emails are removed from the server and will not be visible in Webmail or on other devices. You are also responsible for backing up this local .pst file. However, all items remain fully searchable within Outlook (just ensure search is set to “All Outlook Items”).

  3. Use Outlook’s Built-in Cleanup Tools

    • What it does: Reduces mailbox size without archiving. Go to File > Info > Tools > Mailbox Cleanup.

    • Find large items: Search for emails over a certain size to quickly identify what’s taking up the most space.

    • Clean Up Conversation: This tool removes redundant, quoted messages from long email threads, saving significant space.

Frequently Asked Questions (FAQ)

Q: Is editing the registry safe? A: It is safe if you follow the instructions precisely. The values we are adding only affect Outlook and do not impact the rest of your operating system.

Q: Will a larger data file slow down Outlook? A: Potentially, yes. A very large data file (>75 GB) can sometimes cause slower search results or general sluggishness, especially on older computers with mechanical hard drives (HDDs). If you have a modern PC with a Solid State Drive (SSD), the impact is usually negligible.

Q: What is the difference between an OST and a PST file? A: An OST (Offline Storage Table) file is a synchronized cache of your mailbox from an IMAP or Exchange server. A PST (Personal Storage Table) file is a local archive file used for storing emails and folders directly on your computer. The registry fix works for both.

Conclusion

Hitting Outlook’s 50 GB data file limit can bring your workflow to a halt. By making a simple but powerful adjustment in the Windows Registry, you can expand this limit and get back to work without compromising access to your emails. For best performance, combine this fix with regular mailbox maintenance habits.

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