How to Move Microsoft Outlook OST Data File to Another Drive (IMAP / SMTP)

When the C: drive becomes full, Microsoft Outlook may stop receiving or syncing emails. This is a common issue for users configured with IMAP/SMTP email accounts, where Outlook stores cached mailbox data in an OST (Offline Storage Table) file on the system drive.

This Knowledge Base article explains why this happens, why OST files cannot be moved manually, and provides a safe, supported, and permanent solution to relocate Outlook OST data files to another drive (D:, E:, etc.) using registry-based configuration.

Product Overview 🔗

Microsoft Outlook is an email client included with:

• Microsoft 365 Apps

• Office 2019

• Office 2016

For IMAP accounts, Outlook uses OST files to locally cache mailbox data for performance and offline access.

Key Features Relevant to This Issue 🔗

• IMAP email synchronization

• Cached mailbox storage (OST)

• Local disk dependency for email operations

• Registry-based configuration for data file paths

Technical Explanation 🔗

What is an OST File? 🔗

An OST (Offline Storage Table) file is a local cache of mailbox data stored on the user’s computer. For IMAP and Exchange-based accounts, Outlook automatically manages this file.

Important characteristics:

• OST files are auto-generated by Outlook

• They cannot be attached or added manually

• Deleting an OST does not delete emails from the server

• Outlook recreates the OST during next startup

Why Outlook Stops Receiving Emails 🔗

When the system drive (C:) runs out of free space:

• Outlook cannot write to the OST file

• IMAP synchronization fails

• Incoming emails stop downloading

• Errors or freezes may occur

Use Cases 🔗

This solution is applicable when:

• C: drive is full and cannot be expanded

• Outlook uses IMAP/SMTP account

• OST file size is large (5–50 GB)

• System performance is degraded

• Email sync failures are observed

Step-by-Step Solution (Recommended & Supported) 🔗

Prerequisites 🔗

• Outlook 2016 / 2019 / Microsoft 365

• Administrative access to Windows

• Secondary drive available (D:, E:, etc.)

Step 1: Close Microsoft Outlook 🔗

Ensure Outlook is completely closed.

• Verify using Task Manager

• No Outlook.exe process should be running

Step 2: Create a New Folder on Another Drive 🔗

Example:

Step 3: Open Registry Editor 🔗

Navigate to:

Note: Version numbers

• Outlook 2016/2019/365 → 16.0

• Outlook 2013 → 15.0

• Outlook 2010 → 14.0

Step 4: Create Required Registry Values 🔗

Create two String (REG_SZ) values:

Even for IMAP accounts, ForcePSTPath is required for internal Outlook handling.

Step 5: Delete Existing OST File 🔗

Navigate to:

Delete the existing .ost file associated with the email account.

This is safe for IMAP accounts.

Step 6: Start Outlook 🔗

On startup:

• Outlook creates a new OST file in E:\Outlook

• Email synchronization resumes

• C: drive usage reduces immediately

Verification Steps 🔗

• Check E:\Outlook for newly created .ost file

• Confirm new emails are syncing

• Confirm C: drive free space has increased

Common Issues & Fixes 🔗

Issue: OST still created on C: drive 🔗

Fix:

• Ensure Outlook was closed before registry change

• Confirm registry path version (16.0)

• Delete old OST file again

Issue: Outlook shows sync errors 🔗

Fix:

• Run Account Repair from Outlook

• Restart Outlook

Issue: Registry values not applied 🔗

Fix:

• Ensure values are REG_SZ (String)

• Ensure no trailing spaces in path

Security Considerations 🔗

• Do not store OST files on removable drives

• Avoid network shares for OST storage

• Ensure NTFS permissions are intact

• Backup important PST archives separately

Best Practices 🔗

• Keep at least 10–15 GB free on C: drive

• Limit IMAP sync to 3–6 months if possible

• Archive old emails periodically

• Avoid converting OST to PST unless required

• Monitor OST size regularly

Conclusion 🔗

For IMAP/SMTP email accounts, OST files cannot be moved manually. The only safe and supported method is to configure Outlook using registry keys so that new OST files are created on another drive. This approach permanently resolves disk space issues, restores email synchronization, and aligns with professional IT best practices.

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