I worked for a staffing firm many many years ago. One thing they always made us do was have the candidate come to our office to get their offer letter. Why? Because this is when we got what we wanted. More Names of people to call to place.
How you say.
We called the candidate, told them they got the job. At this point the candidates are so happy they will say YES to almost anything.
We then ask them to bring the most current copy of their Company Employee List as thanks.
It works 99.5 % of the time. Try it next time....