The widely recommended method, is to connect to the Office365 Exchange server via PowerShell. This may need to be done in PowerShell run as administrator; certainly the first line has to be run thusly, although that one line may have to be run only once per client machine.
Set-ExecutionPolicy Unrestricted
(type Y for Yes)
$LiveCred = Get-Credential
(enter your Office365 administrative credentials)
$Session = New-PSSession -ConfigurationName Microsoft.Exchange -ConnectionUri https://ps.outlook.com/powershell/ -Credential $LiveCred -Authentication Basic -AllowRedirection Import-PSSession $Session Set-RemoteDomain Default -TNEFEnabled $false Remove-PSSession $Session