Dear Attorney Zhang,
Hope this email finds you well. In this Thanksgiving season, we would like to THANK YOU for helping us going through the green card application. When I started to prepare for the application, which was four years ago, I was still a Ph.D student (not even a candidate). Before, we had tried to contact other attorneys but was told my case was a little bit weak and it would be better to wait for more time. It was you who was willing to help us. We really appreciate your devotion to making our case succeed.
We wish you a happy holiday.