The new Outlook falls short of meeting the functionality and quality expectations of many businesses, yet Microsoft is pressing organizations to make the switch. To block an unwanted migration to the new client, administrators can configure several Group Policy settings, including one that allows users to revert to the previous version of Outlook.

In early 2024, Microsoft unveiled its three-phase plan for migrating users to the new Outlook. As part of this transition, the new client is set to become the default, though users will still have the option to continue using the classic version for now.

New Outlook drops POP3 and on-prem support

The classic Outlook will remain supported until at least 2029, meaning there is no immediate need to migrate. This is particularly relevant given the new Outlook’s limitations, which provide minimal motivation to switch. One major drawback is the lack of support for on-premises Exchange servers and the POP3 protocol, which poses significant challenges for many organizations.

Microsoft’s latest push affects customers with M365 Business and Premium licenses. Beginning January 6, 2025, these users will be automatically transitioned to the new client. The previously available option to block this migration will not be effective. Instead, Microsoft has introduced a dedicated registry key to manage the migration.

Details of this planned change were shared via the M365 Message Center in a notification with the ID MC926895. However, this message has since disappeared, and no further references can be found on Microsoft’s website, leaving many customers without clear guidance.

Microsoft 365 Enterprise customers reportedly receive an extended grace period, with the automatic migration to the new Outlook set to start in April 2026.

Preventing automatic migration

Regardless of whether the automatic migration begins on January 6 or later, users can safeguard against an unwanted transition by configuring a new Group Policy setting named Manage user setting for new Outlook automatic migration.

It can be found under User configuration => Policies => Administrative templates => Microsoft Outlook 2016 => Outlook options => Other. To enable this setting, ensure you have the latest ADMX files, which can be downloaded here. Setting the value to 2 will effectively prevent the automatic migration to the new Outlook.

In addition to this new setting, another one serves the same purpose. Although Microsoft doesn’t mention the Group Policy setting referenced above in their documentation, they do recommend using a different registry key:
Windows Registry Editor Version 5.00

[HKEY_CURRENT_USER\Software\Policies\Microsoft\office\16.0\outlook\preferences]
"NewOutlookMigrationUserSetting"=dword:00000000
Given these inconsistencies, configuring both settings as a precaution is advisable. If you need to deploy the registry value via Group Policy, follow the steps outlined below to roll back the migration.

Blocking admin-initiated migration

In addition to blocking the Microsoft-initiated switch to the new Outlook, you can also prevent a migration triggered by an administrator. The corresponding setting is Admin-Controlled Migration to new Outlook.

While this policy blocks migrations initiated by administrators, it does not prevent Microsoft’s automatic migration. For that, the previously mentioned setting must be used. Notably, the registry key name DoNewOutlookAutoMigration might allow for alternative interpretations.

Additionally, it is recommended to disable the toggle in the classic Outlook that permits users to switch to the new version. This can be achieved using the following setting:

• Hide the Try the new Outlook toggle in Outlook

All Office settings can generally be configured via cloud policies. However, the new option to block automatic migration is currently unavailable in the cloud environment.

Reverting migration

Users who switched to the new Outlook using the toggle can revert to the classic client with a registry key. Although no specific Group Policy exists for this, you can achieve the same result by using Group Policy Preferences (GPP).

Follow these steps to proceed:

1. Create a new GPO for the organizational units (OUs) containing the relevant user accounts
2. Navigate to: User Configuration => Preferences => Windows Settings => Registry
3. Execute the command: New => Registry Item

In the dialog that appears, fill in the fields as follows:

• Hive: HKEY_CURRENT_USER
• Key Path: Software\Microsoft\Office\16.0\Outlook\Preferences
• Value name: UseNewOutlook
• Value type: REG_DWORD
• Value data: 0

Conclusion

Microsoft announced plans to migrate business customers to the new Outlook starting in January. However, many organizations will not welcome this migration due to the limitations of the new client. To prevent the migration, administrators can use Group Policy.

Additional settings are available to block migrations initiated by administrators or to reverse an unwanted transition to the new client.

For complete peace of mind, it is recommended to configure all available settings to prevent any undesired migration entirely.