This one specifically affects people using classic Outlook for Windows (not the new Outlook) with POP email accounts, where the PST or OST data file lives inside a folder synced by OneDrive (or similar cloud storage like Dropbox). If that's your setup and Outlook started freezing, hanging on close, or silently failing to show sent emails after a recent Windows update, this is a confirmed, Microsoft-acknowledged issue — not a sign that your mailbox or PC is broken.

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What's actually happening

The January 13, 2026 Windows update KB5074109 changed how Windows handles applications reading from and writing to cloud-backed storage like OneDrive. With POP accounts specifically, classic Outlook writes everything — including sent messages — directly into the PST file on exit. If that PST sits inside a OneDrive-synced folder, OneDrive's sync engine can grab or hold a lock on the file while it's scanning or uploading changes in the background.

When Outlook then tries to write to that same file on close, it ends up waiting on OneDrive to release the lock — but the wait doesn't resolve cleanly, so Outlook hangs instead of closing, and the OUTLOOK.EXE process keeps running in the background even after the window appears to have closed. Because the write to Sent Items often doesn't complete before this happens, sent emails can fail to appear in the Sent Items folder even though they were genuinely sent.

The immediate fix when it happens

If Outlook is currently frozen or won't close:

  1. Open Task Manager (Ctrl+Shift+Esc)
  2. Find OUTLOOK.EXE in the list — there's a good chance it's still running even if the window itself is gone
  3. Select it and click End Task
  4. Reopen Outlook

This clears the stuck process, but it's a per-incident fix, not a permanent one — the same freeze can recur next time you close Outlook, since the underlying conflict between OneDrive sync and the PST write hasn't been resolved.

The real fix: move your PST/OST file out of OneDrive

The most reliable permanent fix is to move your Outlook data file out of any cloud-synced folder entirely:

  1. In Outlook, go to File → Account Settings → Account Settings
  2. Click the Data Files tab to see where your current PST/OST file is stored
  3. Close Outlook completely (check Task Manager to confirm OUTLOOK.EXE isn't still running)
  4. Move the PST/OST file from its OneDrive-synced location to a local-only folder — for example, directly under C:\Users\[YourName]\Documents\Outlook Files rather than inside a folder OneDrive is backing up
  5. Reopen Outlook and reconnect the data file to its new location if it doesn't pick it up automatically (Account Settings → Data Files → Open File Location, then point it to the new path)

Why this works: the conflict only happens because OneDrive is actively managing (scanning, syncing, sometimes locking) the file at the same time Outlook needs exclusive write access to it. Once the PST/OST lives in a purely local folder that OneDrive isn't touching, there's no longer a competing process trying to access the same file, and Outlook can write and close normally.

Temporary workaround if you can't move the file right now

If moving the data file isn't immediately practical, Microsoft's guidance is to use Task Manager to force-close OUTLOOK.EXE whenever it hangs on exit, accepting that sent emails may occasionally need to be manually re-checked. This isn't a great long-term routine, but it keeps things usable until you can do the proper fix above.

If you're not comfortable making changes to where your data files live, or you're managing this across multiple machines in an office, it may be worth pausing Windows Updates temporarily (Settings → Windows Update → Pause updates) until you've had a chance to migrate everyone's PST files out of OneDrive-synced folders in a controlled way, rather than hitting this on each machine as it updates individually.