When Microsoft rolls out the new Outlook for Windows, power users often need to test it without losing access to the classic version. Running both versions side by side lets you compare features, verify add-in compatibility, and confirm that your daily workflows still work. This article explains the only supported method to install and run both versions on the same machine, the limitations you must know before starting, and the steps to switch between them safely.
Key Takeaways: Running Classic and New Outlook Side by Side
- Toggle switch in the new Outlook title bar: Lets you turn the new Outlook on or off without uninstalling it.
- Separate profile and data storage: New Outlook uses a modern cloud-based profile; classic Outlook keeps its own offline data files.
- No add-in or COM add-in support in new Outlook: Classic Outlook remains necessary for legacy add-ins or custom forms.
Why Running Both Versions Is Supported but Has Limits
Microsoft designed the new Outlook for Windows as a replacement for the classic desktop client. The toggle that enables the new Outlook appears automatically for Microsoft 365 subscribers on Current Channel builds. When you turn on the toggle, the new Outlook installs and becomes the default mail app. Classic Outlook remains installed, but its shortcut may be hidden.
The side-by-side state is temporary. Microsoft intends the toggle as a preview mechanism, not a permanent dual-client solution. You can switch between the two versions any number of times, but each switch restarts the app and may require reconfiguring certain settings.
The main technical difference is how each version stores data. Classic Outlook uses local PST or OST files and supports offline access to Exchange mailboxes. New Outlook uses a cloud-connected profile that syncs directly with Exchange Online or Microsoft 365. Because the two clients do not share a single data store, changes you make in one version do not appear in the other until the server sync completes.
Steps to Enable and Switch Between Classic and New Outlook
Follow these steps to install the new Outlook while keeping the classic version available. The process assumes you already have classic Outlook installed as part of Microsoft 365 Apps for enterprise or business.
- Open classic Outlook and locate the toggle
Launch Outlook the way you normally do. Look at the top-right corner of the window. You should see a toggle labeled Try the new Outlook. If you do not see it, your organization may have disabled the preview. Contact your IT administrator or check the Microsoft 365 admin center for the New Outlook policy setting. - Turn on the toggle to install the new Outlook
Click the toggle to the On position. A dialog appears explaining that the new Outlook will open after the switch. Click OK. The new Outlook installs automatically. No separate download is required. After installation completes, the new Outlook window opens with your primary mailbox loaded. - Verify both versions are present on the system
Open the Start menu and type Outlook. You should see two entries: Outlook (new) and Outlook (classic). If only one appears, the classic shortcut may have been hidden. To restore it, go to Control Panel > Programs and Features, right-click Microsoft 365 Apps, and select Change. Choose Quick Repair and follow the prompts. This restores all default shortcuts. - Switch back to classic Outlook from the new version
In the new Outlook window, click the Try the new Outlook toggle again to turn it Off. A confirmation dialog asks if you want to keep the new Outlook installed. Select Keep installed if you plan to switch again later, or Uninstall to remove it. Classic Outlook opens immediately after you confirm. - Test your workflows in each version
Open the same email, calendar item, or contact in both versions. Check that all data appears correctly. Pay attention to shared mailboxes, public folders, and delegate permissions. Classic Outlook supports these features natively. New Outlook may require additional configuration or may not support them at all.
What Happens to Settings, Signatures, and Rules
When you switch between versions, some settings do not carry over. Signatures created in classic Outlook do not appear in the new Outlook. You must recreate them manually. The same applies to Quick Steps, custom views, and most third-party add-ins.
Email rules created in classic Outlook are stored on the Exchange server. They work in both versions because the server executes them. Rules that depend on client-side actions, such as moving items to a local PST file, run only in classic Outlook.
Account settings also differ. Classic Outlook stores account configurations in the Windows Registry under HKEY_CURRENT_USER\Software\Microsoft\Office\16.0\Outlook\Profiles. New Outlook stores its configuration in the Microsoft 365 cloud. If you add a new account in one version, you must add it again in the other.
Limitations and Workarounds When Running Both Versions
New Outlook crashes when opening a shared mailbox
Shared mailboxes that require explicit permissions may not load in the new Outlook. Open the shared mailbox in classic Outlook first to verify that permissions are correct. If it works in classic, the issue is likely a new Outlook limitation. Use classic Outlook for that mailbox until the new version adds full support.
Classic Outlook prompts for password repeatedly after switching
The new Outlook changes your default authentication method to modern auth. Classic Outlook may lose its cached credentials. Open Control Panel > Credential Manager > Windows Credentials and remove any entries that include Outlook or MicrosoftOffice16. Restart classic Outlook and sign in again.
Calendar items appear duplicated in both versions
Because each version maintains its own local cache, you may see duplicate calendar entries if you create an appointment in one version and then sync fails. Wait for the server sync to complete, then close and reopen the calendar view. If duplicates persist, delete the extra entry from the version that did not create it.
Classic Outlook vs New Outlook: Side-by-Side Comparison
| Item | Classic Outlook | New Outlook |
|---|---|---|
| Data storage | Local PST/OST files | Cloud-connected Microsoft 365 profile |
| Add-in support | Full COM add-in and VBA support | Web add-ins only, no COM support |
| Offline access | Full offline mode with cached Exchange | Limited offline capabilities |
| Public folders | Fully supported | Read-only support in preview |
| Shared mailbox delegation | Full delegate permissions | Basic shared mailbox access |
| Custom forms | Supported | Not supported |
| Quick Steps | Supported with full customization | Not supported |
| Signatures | Stored locally, fully editable | Stored in cloud, basic editor |
You can run classic Outlook and the new Outlook side by side using the toggle switch. This method gives you a safe way to evaluate the new client without losing access to legacy features. Before switching permanently, recreate your signatures and Quick Steps in the new Outlook. If your workflow depends on COM add-ins or public folders, keep classic Outlook as your primary client until Microsoft adds full support for those features in the new version.