Dear Mr. and Mrs. Smith,
I'm writing this letter to you to sincerely say "thank you". I really appreciate your kind assistance and strong support during my stay in your city. Without your help, it would have been impossible for me to finish my job there so smoothly and successfully.
Now that I'm back in Tianjin, I would like to invite both of you to visit my home city at a convenient time. You are most welcome to stay at my home for a couple of weeks, and I will be more than glad to be your local tour guide.
Look forward to meeting you here in Tianjin,
Yours sincerely,
XXX
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