Outlook Data File Reached Maximum Size: Fix the PST Size Limit in 2026

Outlook data files (PST/OST) have a 50 GB limit for modern UNICODE format PST files (Outlook 2003–2007 ANSI format PST: 2 GB limit). When the limit is reached, Outlook displays "The data file has reached the maximum size" and may stop receiving new emails.

Part 1. Understanding PST File Size Limits

💡 Tip: To check which PST format you're using: File → Account Settings → Data Files → right-click the PST → Properties. If it shows "Outlook Data File" (not "Personal Folders"), it's UNICODE format with the 50 GB limit.

Part 2. Archive Old Emails to Reduce PST Size

Archiving moves old emails to a separate PST file, freeing space in the main file:

1. File → Tools → Clean Up Old Items (or Archive in some versions)
2. Set archive date (e.g., "Archive items older than 1 year")
3. Choose a location for the archive PST file
4. Click OK — old emails move to the archive file

Alternatively, manual archive:

1. Right-click any folder → Properties → AutoArchive tab
2. Configure per-folder archiving settings

⚠️ Important: Archived emails are in a separate PST file — they're not deleted. You can access them in Outlook by opening the archive file: File → Open & Export → Open Outlook Data File → select the archive PST.

Part 3. Compact the PST File

After deleting or archiving emails, the PST doesn't automatically shrink — you need to compact it:

1. File → Account Settings → Data Files
2. Select the PST → Settings
3. Click Compact Now
4. Wait for completion (may take several minutes for large files)

💡 Tip: The Compact Now process can take a long time for large PST files — leave Outlook running and don't close it. A 40 GB PST may take 30–60 minutes to compact significantly.

Part 4. Increase the PST Size Limit via Registry

For situations where 50 GB isn't enough:

1. Press Win + R → type regedit → Enter

2. Navigate to:

   HKEY_CURRENT_USER\SOFTWARE\Microsoft\Office\16.0\Outlook\PST
   

3. Create new DWORD values:
   • MaxLargeFileSize — maximum size in MB (e.g., 102400 = 100 GB)
   • WarnLargeFileSize — warning threshold in MB (e.g., 92160 = 90 GB)
4. Restart Outlook

💡 Tip: Increasing the registry limit to 100 GB buys time but doesn't solve the underlying problem. Large PST files are slower, more prone to corruption, and harder to back up. Plan a proper archiving strategy alongside the registry change.

🗣️ r/sysadmin guidance: "For corporate users hitting the PST limit, the best long-term solution is archiving to Exchange Online or an archive mailbox — not just increasing the limit. Huge PST files are slow, fragile, and hard to back up."

🗣️ r/techsupport user: "PST hit 50 GB limit and Outlook froze constantly. Archived everything older than 2 years, compacted the PST, and it's now under 20 GB and fast again. Should have done it sooner."

FAQ

What happens when the Outlook data file reaches maximum size? Outlook slows significantly, may stop receiving new emails, show errors when saving drafts, and eventually prevent sending or receiving until the file is reduced below the limit.

How can I tell how large my PST file is? File → Account Settings → Data Files → the PST file path is shown. Navigate to that path in File Explorer to check the file size.

Is archiving emails the same as deleting them? No — archiving moves emails to a separate PST archive file. They're still accessible in Outlook by opening the archive file. Archiving reduces the main PST size without losing email history.

What's the difference between compacting and reducing a PST file? Compacting reclaims "white space" left by deleted or moved emails — the PST file physically shrinks to match its actual content. Simply deleting emails doesn't reduce file size until you compact.

References

• Microsoft — Manage PST File Size
• Microsoft — Compact Outlook Data Files
• r/sysadmin — PST Size Limit Management
• r/techsupport — PST Archive Fix
• r/MSOffice — PST Registry Limit