Classic Outlook freezing on launch, stuck on "Not Responding," or silently dying in the background after a Windows update? The January 13, 2026 cumulative update (KB5074109) broke something. If your PST files live inside OneDrive or another cloud-synced folder, Outlook hangs every time it touches them.

Microsoft confirmed the bug: apps become unresponsive when opening or saving files in cloud-backed locations. PSTs in a synced folder cause freezes on launch, sent items vanish, and previously downloaded mail re-downloads itself.

Who Gets Hit

  • Windows 10 and Windows 11 machines with KB5074109 installed
  • Classic Outlook desktop (Microsoft 365 Apps or perpetual Office) with PSTs stored in OneDrive, Dropbox, or another synced folder
  • Systems missing the out-of-band fix KB5078127

If you're on the new web-style Outlook that doesn't use local PST files, this isn't your problem.

Before You Start

  1. Make sure you have 10-15 GB free on your system drive.
  2. Close Outlook. Open Task Manager and kill any stray OUTLOOK.EXE processes.
  3. Create C:\PST-Backup\ and copy your PSTs there first.

The Fix

Method 1: Move PST Files Out of OneDrive

This is the big one. Active PST files don't belong in OneDrive. That's been true for years; this update just made it obvious.

Find your PSTs:

Open Outlook. Go to File > Account Settings > Account Settings. Open the Data Files tab and note the full path for each PST.

Move them:

  1. Close Outlook.
  2. Create C:\OutlookData (make sure it's not inside any sync folder).
  3. Move each PST into C:\OutlookData.

Repoint Outlook:

Open Outlook. If it complains about a missing data file, browse to C:\OutlookData and select the PST. If it opens fine, go to File > Account Settings > Data Files, click Add, point to the moved PST, and Set as Default if needed.

Many freezes stop here, even before the Windows fix.

If That Doesn't Work:

Method 2: Install KB5078127

Microsoft shipped an emergency out-of-band patch for this bug.

  1. Open Settings > Windows Update and check for updates.
  2. Look for KB5078127 (Windows 11) or equivalent for your OS.
  3. Install and restart.

If it's not showing up, grab it from the Microsoft Update Catalog manually.

If That Doesn't Work:

Method 3: Repair or Recreate Your Outlook Profile

The profile probably got corrupted during earlier crashes.

Repair:

  1. Close Outlook.
  2. Open Control Panel. Search Mail, open Mail (Microsoft Outlook).
  3. Click Show Profiles, select your profile, click Properties, then Email Accounts.
  4. Highlight each account, click Repair.

If repair fails, create a new profile:

  1. Same Mail dialog. Click Add, create a new profile.
  2. Add your accounts. Point data files to C:\OutlookData.
  3. Set Always use this profile with the new one as default.

If That Doesn't Work:

Method 4: Pause OneDrive Sync (Temporary)

If you can't move PSTs yet, at least pause OneDrive while Outlook runs.

  1. Click the OneDrive icon in the system tray.
  2. Gear icon, Pause syncing, pick a time.
  3. Resume after closing Outlook.

Band-aid. Move the PSTs when you can.

Quick Reference

Outlook hangs on startup -- PST in OneDrive + KB5074109 + no patch. Move PST (Method 1), install KB5078127 (Method 2).

Freezes when sending/receiving -- PST conflict or profile damage. Confirm PST is local, repair profile (Method 3).

Sent items missing, mail re-downloading -- PST corrupted mid-crash. Move PST, apply updates, let Outlook re-sync. Create new PST if needed, keep old one as archive.

PC sluggish when Outlook runs -- Missing KB5078127 + cloud-backed PSTs. Install the fix, move PSTs to local drive.

For IT Admins

  • Block PSTs in OneDrive via Group Policy where possible.
  • Standardize C:\OutlookData in deployment images.
  • Use online archives or shared mailboxes instead of giant PSTs.
  • Audit for machines on KB5074109 without KB5078127.

Need hands-on help? Contact Rain City Techworks and mention the Outlook / KB5074109 issue.