[BlueOnyx:26341] Re: Backscatter / user not found bounce

[Colin Jack]

Hi Michael,

2.) Enable and configure SPF and switch it to "Sign & Verify" mode

This checks the SPF records of sender domains and if the senders
IP is not within the SPF records published by say hotmail.com, then
the email will be rejected at the MTA w/o bounce and NDN.

This may be a doh! question but where do I set this? 
I can only see DKIM in the settings on the GUI.

Thanks

Colin