Outlook email will not open.

Do not uninstall the listed updates yet; first identify which Outlook app is installed and repair that client, because Microsoft has not documented those updates as causing this specific Outlook error on Windows 11 version 26H2.^1^

1. Identify and repair Outlook

Open Start > Settings > Apps > Installed apps, then search for Outlook.

If it says “Outlook (new)” or “Outlook for Windows”

1. Select the three dots beside Outlook.
2. Select Advanced options.
3. Select Terminate, and then Repair.
4. Open Outlook and test it.
5. If it still displays “page could not be reached,” return to the same page and select Reset.

Resetting removes the app’s local data and requires you to sign in again, but server-stored email remains in the mailbox. Windows documents Repair, followed by Reset when repair does not work.^2^

Microsoft’s current new-Outlook issue list does not show this error as a published issue associated with the updates you named.^3^

If it says “Microsoft 365,” “Microsoft Office,” or classic “Outlook”

First test classic Outlook without add-ins:

1. Press Windows key + R.
2. Enter:

      outlook.exe /safe
   

3. Select OK and accept the default profile.
4. If Outlook opens, go to File > Options > Add-ins.
5. Beside Manage: COM Add-ins, select Go.
6. Clear the add-in checkboxes and restart Outlook normally. Re-enable them individually to identify the problem add-in.^4^

If safe mode does not open:

1. Right-click Start > Installed apps.
2. Find Microsoft 365 or Microsoft Office.
3. Select three dots > Modify.
4. Run Quick Repair.
5. If that fails, repeat the process and choose Online Repair. Online Repair is more comprehensive and requires an internet connection.^5^

2. Check whether the problem is limited to the app

Open your normal web browser and sign in to Outlook on the web using your usual Microsoft account or work/school account.

• If mail opens there, the mailbox and connection are working; continue repairing the Windows Outlook app.
• If the web page also fails, record the complete error wording and try another browser. For a work or school account, contact the organization’s Microsoft 365 administrator if the failure persists.

3. Uninstall an update only as a controlled fallback

The update list does not establish which update—if any—caused Outlook to fail:

• KB5007651 is a Windows Security platform update.
• KB890830 is the Malicious Software Removal Tool.
• The Windows 11 26H2 release-health page does not list your Outlook symptom against these updates.^1^

If Outlook still fails after the applicable repair, you can test removal of the most recent removable quality update:

1. Open Start > Settings > Windows Update.
2. Select Update history.
3. Under Related settings, select Uninstall updates.
4. Note or photograph the update list first.
5. Remove only the latest update that Windows offers as uninstallable.
6. Restart the laptop and test Outlook.

Some updates cannot be uninstalled. Removing a security update reduces protection, so avoid removing several updates together; otherwise, it becomes difficult to determine which one affected Outlook.^6^

If uninstalling the latest removable update does not change the behavior, reinstall available updates through Settings > Windows Update rather than removing the remaining security components.

References

1. Windows 11, version 26H2 known issues and notifications
2. Windows 11
3. Fixes or workarounds for recent issues in new Outlook for Windows ...
4. I can’t start Microsoft Outlook or receive the error “Cannot start Microsoft Outlook. Cannot open the Outlook Window” | Microsoft Support
5. Windows 11
6. Windows 11