PipeLineTracing in Exchange 2013 fails to write output

I am troubleshooting an issue with email flow in Exchange 2013.  I have enabled PipelineTracing and all parameters

Set-TransportService Mailbox01 -PipelineTracingSenderAddress chris@contoso.com
Set-TransportService Mailbox01 -PipelineTracingPath "F:\Anti-Spam_Agents_Log"
Set-TransportService Mailbox01 -PipelineTracingEnabled $true 
Get-TransportService Mailbox01 | Format-List PipelineTracing*

With the last command it reports all is configured correctly but... when I send an email from the mailbox nothing is written to the output folder.  I can't see any details of required folder permissions (set it to everyone FC as a test but no change) for Exchange to write to the chosen output folder.

I also stopped and restarted the Transport and MBX Transport Delivery services just in case.

What have I missed, any suggestions