Microsoft has unilaterally imposed the ‘Clutter’ feature on all users of Office 365. If you want to disable this feature for all users in the domain, follow these instructions.

1. Open Power Shell and run the following command:
   $UserCredential = Get-Credential
   Then enter your (administrator) Office365 credentials
2. Run the following command (all on one line):
   $Session = New-PSSession -ConfigurationName Microsoft.Exchange
   -ConnectionUri https://outlook.office365.com/powershell-liveid/ -Credential
   $UserCredential -Authentication Basic -AllowRedirection
3. Run the following command:
   Import-PSSession $Session
4. Run the following command to turn off Clutter:
   Get-mailbox -ResultSize Unlimited | Set-Clutter -Enable $false
   
5. Now remove the imported session with this command:
   Remove-PSSession $Session