How to disable the New Outlook for yourself or whole Microsoft 365 Tenant 

The new Outlook version is a big change compared to the classic Outlook desktop. The preview version might work well for some, well others may still be missing important features that they really need for their daily work.

When you are a Microsoft Insider, then you will see the Try the new Outlook toggle in the top-right corner of the Outlook or Mail app. Good to know, is that you can always switch back to the classic Outlook version using the same toggle. This way you can try out the new features, and in case you are missing important features, easily switch back when needed.

The new version of Outlook will download after you have switched the toggle. It takes only a minute to download it after which the migration wizard will start. Outlook will import most settings from the classic Outlook version. All the settings will also automatically sync to Outlook Online after the import.

Missing features

As mentioned, the app is still in development. Some features are missing at the moment, but are planned or being developed right now. Below is a short list of the most important features that are missing at the moment and their know status:

• Unable to save email as .eml file.
• Multi-account support – Is being developed right now.
• Offline access – In development. Currently, you can only use the new Outlook App with an Internet connection
• POP Support – Microsoft is investigating if they are going to add POP support
• Search Folder – In development
• Folder reordering – In development
• Send/receive now – Unkown. Internet connection is required now, so the feature is not really needed at the moment. Might come back with offline support?
• Downloading attachments – You can’t choose the folder anymore. Downloaded attachments are stored in your downloads folder
• Find related message – Right-clicking on an email and finding related messages is not possible.
• Recall email – You can’t recall an email anymore. This feature is somewhat replaced by the undo send feature.

How an end user can switch back to previous Outlook

Toggle the "New Outlook" in top right corner to off.

Wait for Outlook to Restart

How to Disable the New Outlook for your Tenant 

If your users are part of the Office insider channel, then they can enable the new Outlook App. You can prevent this with the following PowerShell command:

# Connect to Exchange Online first
Connect-ExchangeOnline

# Disable the new Outlook app for all users:
Get-CASMailbox | Set-CASMailbox -OneWinNativeOutlookEnabled $false

# Or for a single user:
Set-CASMailbox -identity johndoe@contoso.com -OneWinNativeOutlookEnabled $false

# Verify change
Get-CASMailbox | ForEach-Object {
Write-Host "$($_.DisplayName) - OneWinNativeOutlookEnabled: $($_.OneWinNativeOutlookEnabled)"
}

source: https://lazyadmin.nl/office-365/new-outlook-for-windows/#missing-features-and-important-changes