Simply replace the unfilled areas such as "[]" and "[date]" with your information.
Dear [name],
Because [name of absentee] is out of the office for the next [period of time], I am acknowledging receipt of your letter dated [date]. It will be brought to [his/her] attention immediately upon [his/her] return.
If I may be of any assistance during [name of absentee] absence, please do not hesitate to call or write.
Best regards,