"Dear Attorney Zhang,

I hope this email finds you well. 

I would like to send my great gratitude to your law firm, especially Attorney Bennett Wisniewski. Because of his outstanding expertise and dedication, my immigration case has been successful. I just moved to the US yesterday. I can't thank you enough for all the great work and help that attorney Ben has given me throughout the whole process. I believe all your law firm's attorneys are talented, but I still feel very lucky that you assigned my case to attorney Ben. 

In this email, I also would like to refer my colleague to your firm for NIW application. Hope you would help XXXX first with an initial evaluation. 

I look forward to hearing from you.