The latest KB5074109 update for Windows 11 causes the classic Outlook app to crash and lose emails, and there’s a risk of data loss if PST or OST files are stored on cloud-backed storage! You can read more about it on this Microsoft support page.
If classic Outlook crashes due to the issue caused by update KB5074109, you must close the email program via Task Manager and restart it. This may result in sent emails not appearing in the Sent Items folder.
Microsoft writes:
After installing Windows updates released on or after January 13, 2026 (KB5074109), some applications might become unresponsive or experience unexpected errors when opening files from or saving files to cloud-backed storage, such as OneDrive or Dropbox.
For example, in some configurations of Outlook that store PST files on OneDrive, Outlook might become unresponsive and fail to reopen unless its process is terminated in Task Manager, or the system is restarted. In addition, sent emails might not appear in the Sent Items folder, and previously downloaded might be downloaded again.
Elsewhere, Microsoft also writes:
After Windows updates on January 13, 2026 users with Outlook POP account profiles and profiles with PST files report that Outlook hangs and does not exit properly. This issue may occur for any Outlook profile that has PSTs stored on OneDrive.
Symptoms reported include:
- Outlook hangs and shows “Not Responding.”
- Inability to reopen Outlook without ending its process in Task Manager or restarting the computer.
- Emails not appearing in the Sent Items folder despite being sent.
- Outlook redownloading emails.
Affected platforms:
- Windows 11, version 25H2; Windows 11, version 24H2; Windows 11, version 23H2; Windows 10, version 22H2; Windows 10 Enterprise LTSC 2021; Windows 10 Enterprise LTSC 2019
- Windows Server 2025; Windows Server, version 23H2; Windows Server 2022; Windows Server 2019
Until a fix is available, please use webmail. The other workarounds may be complicated.
Microsoft’s offered workaround
According to BleepingComputer, Microsoft has shared the following advice for affected users:
- If this problem occurs, please contact the application developer to find out about possible alternative methods for accessing the files.
- For Outlook-specific scenarios, moving the PST files from OneDrive should resolve the issue. Instructions can be found here.
- In addition, email accounts can still be accessed via webmail, provided this is supported by your email provider.
- Businesses and IT administrators who need urgent help resolving the issue should contact Microsoft Business Support.
Microsoft is working on a patch for the Outlook issue. Until it’s released, you will need to use the workaround described by Microsoft.
Update KB5074109 is the first Windows 11 patch of 2026 and it’s already wreaking havoc on many Windows 11 computers. We’ve reported on the issues in this update, including PCs no longer shutting down, black screen crashes, and File Explorer ignoring some settings.