If your Preno emails are not being received by your Booking.com guests, you need to update your Booking.com extranet settings to assist with this.
In Booking.com, click "Property" then "Messaging Preferences".
Go to 'Security Settings' and make sure '[email protected]' is added to the approved list for contacting guests
This should allow all Preno emails to be sent through to Booking.com guests via the Booking.com extranet.