Dear Sir or Madam,
With reference to our email dated (Datum) it seems that we have not received your comment. May we ask you to look into the matter and tell us which number should be used as supply note number in order to avoid any possible confusion in the future.
We thank you in advance for your urgent reply and remain at your disposal for any further query you may have.
Yours faithfully oder Kind regards,
Wenn Du die Person mit Namen ansprichst z. B. Dear Mr. Müller, dann must Du yours sincerely oder Kind regards nehmen