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It should start with thanks giving words like i am thankful to you for providing me this kind of opportunity in your organization. You should use polite words in the mail keeping in mind it is a professional world. You should not have bad impression on anyone.
If HR contact you over the phone then update them accordingly or if you received an e-mail then reply them firmly and swiftly so that your future employment opportunity is not missed!
Say only " Thanks to your Reply"