The email can go something like this:
I wanted to thank you again for taking the time to interview with me about this position. ...blah blah....
I would also like to take this opportunity to further clarify my visa status. .... (如果你有幫你辦綠卡的移民律師,你可以把他/她的聯係電話附上,說明in case they have questions please contact this lawyer.)
Email 最好簡潔明了,不要太長(尤其不要解釋很多細節),just make your point.
Good luck -- hope you get that offer.