Classic Outlook removed from Microsoft 365 Family and new Outlook breaks IMAP

Classic Outlook is still available, but it is no longer included with Microsoft 365 Family as part of the default consumer experience. For home users on new Windows PCs, Microsoft is directing them to the new Outlook for Windows, which is a different app and does not yet support all the same scenarios as classic Outlook.

For IMAP and other advanced scenarios that are not working in new Outlook, the supported options from the available information are:

1. Use classic Outlook where it is still supported
   • Classic Outlook for Windows continues to exist and is required for features that new Outlook does not yet support (for example, MAPI-based integrations and some add-ins).
   • On devices where classic Outlook is installed, admins can control whether users are started in classic or new Outlook by using the UseNewOutlook registry key. Setting UseNewOutlook to 0 under HKEY_CURRENT_USER\Software\Microsoft\office\16.0\outlook\preferences keeps users in classic Outlook instead of automatically switching them to the new Outlook experience.
   • If classic Outlook is still present on the machine, switching back from new Outlook is supported via the in-app toggle described in the admin guidance. This allows continued use of classic Outlook with IMAP accounts.
2. Install or reinstall classic Outlook where available
   • For users who should still have access to classic Outlook (for example, via certain Microsoft 365 or Office licenses), Microsoft provides store-based installation paths and guidance in articles such as “You can't open classic Outlook on a new Windows PC” and “Install or reinstall classic Outlook on a Windows PC.” These are referenced from the article about the “go back to classic Outlook” button.
   • If the “go back to classic Outlook” button in new Outlook does not correctly open the support article or install classic Outlook, Microsoft documents this as a known issue that has been fixed, and provides a direct Microsoft Store install link for classic Outlook in that article as a workaround.
3. Use an Outlook version that supports modern authentication and full account connectivity
   • For Outlook.com and Microsoft 365 mailboxes, Microsoft requires modern authentication (OAuth). Older Outlook versions and POP/IMAP configurations that rely on Basic authentication are being deprecated and can lose connectivity.
   • For Outlook.com accounts specifically, Microsoft recommends either:
     • Creating a new Outlook profile and adding the account using the Outlook.com/Exchange configuration (not POP/IMAP), or
     • Moving to a newer Outlook app that supports OAuth, including the new Outlook for Windows.
4. When new Outlook cannot meet requirements
   • New Outlook is not yet MAPI-compliant and cannot fully replace classic Outlook in all scenarios (for example, mail merge and some “Share to email” flows from Word/Excel/PowerPoint still depend on classic Outlook).
   • Where business- or feature-critical workflows depend on classic Outlook capabilities (including some IMAP scenarios), the documented guidance is to continue using classic Outlook as the default mail client instead of new Outlook until the needed functionality is available in the new app.

From the available documentation, there is no supported way to force new Outlook to behave exactly like classic Outlook for IMAP today. The supported path for full IMAP and advanced Outlook features is to use classic Outlook where it is still licensed and installable, and to keep new Outlook disabled or secondary on that device.

References:

• Go back to classic Outlook button not working correctly in new Outlook
• Start new users in new Outlook
• Outlook and other apps are unable to connect to Outlook.com when using Basic authentication
• Upcoming changes to Outlook.com may impact some legacy versions of Outlook for Windows
• Troubleshoot Outlook email setup