What's the correct syntax for recipients in Sprout Email? I've got a form creating an entry in a Bookings channel which contains two fields, an Entries field (eventBooking) and a Users field (userBooking).

I'm trying to get Sprout Email to send an email to the user selected in userBooking. Can anyone help me with the correct syntax please?

I've tried the following which don't seem to work:

{{ userBooking.first().email }}
{{ object.userBooking.first().email }}
{{ entry.userBooking.first().email }}
{ object.userBooking.first().email }
This should work:
{{ object.userBooking.first().email }}

I would check if the problem is with something else.