Thank you very much for choosing YYY(your company's name).
This is to inform you that a letter was sent to your attention but it was returned. Please confirm your mailing address by return e-mail and we will mail it to you again. We will also inform you when the letter is sent next time.
Please be informed that a letter was returned to us as nobody signed for it. Can you confirm the address, and I will notify you as soon as I send out the letter.