Disable the New Outlook for your Tenant

# 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 joemama@contoso.com -OneWinNativeOutlookEnabled $false

# Verify change - Change to a 1 line entry
Get-CASMailbox | ForEach-Object {
Write-Host "$($_.DisplayName) - OneWinNativeOutlookEnabled: $($_.OneWinNativeOutlookEnabled)"
}