cant start outlook classic

In new Outlook, select the toggle in the upper-right and choose Switch back to classic Outlook; this makes Outlook Classic the version opened on subsequent launches.^1^

Switch back to Outlook Classic

1. Open the Outlook app that currently works.
2. Select the New Outlook toggle in the upper-right.
3. Choose:
   • Switch back to classic Outlook to make Classic the default, or
   • Use classic Outlook this time to open it once without changing the default.
4. Open Start, search for Outlook (classic), and select that exact entry—not the Outlook icon marked New.

You can pin Outlook (classic) to Start or the taskbar so it remains easy to identify.^2^

If the toggle is missing

First search the Start menu for Outlook (classic).

• If it is not listed: Classic Outlook might not be installed. On Microsoft’s switch-back page, use the You can’t open classic Outlook on a new Windows PC installation link.^1^
• If it is listed but always redirects to new Outlook: change the documented preference below.

Editing the registry incorrectly can affect Office behavior, so change only this value:

1. Close both Outlook applications.
2. Open Start, type Registry Editor, and open it.
3. Navigate to: Computer\HKEY_CURRENT_USER\Software\Microsoft\Office\16.0\Outlook\Preferences
4. Double-click UseNewOutlook.
5. Set Value data to 0, then select OK.
6. From Start, launch Outlook (classic).^1^

If Classic starts but displays an error or immediately closes

That is a startup problem rather than redirection to new Outlook.

1. Press Windows key + R.
2. Enter Outlook /safe and select OK.
3. If Outlook opens in safe mode:
   • Go to File > Options > Add-ins.
   • Beside Manage: COM Add-ins, select Go.
   • Record the enabled add-ins, then clear their checkboxes. Do not remove them.
   • Restart Outlook normally and re-enable one add-in at a time to identify the conflicting one.
4. If safe mode also fails, create a new Outlook profile:
   • Open Control Panel > Mail > Show Profiles.
   • Select Add, configure the email account, and start Outlook with the new profile.

Microsoft also provides a Classic Outlook startup troubleshooter for Windows 10 or later. Removing an old profile deletes its offline cached content, so do not remove it until any required local data is backed up.^3^

References

1. Toggle out of the new Outlook for Windows | Microsoft Support
2. Run new Outlook and classic Outlook side-by-side | Microsoft Support
3. I can’t start Microsoft Outlook or receive the error “Cannot start Microsoft Outlook. Cannot open the Outlook Window” | Microsoft Support