Dear…
RE: Claim __(Reference Number/Name)__
Please accept this as acknowledgement that we are in receipt of your letter dated __(date)__. We are sorry to hear of the dissatisfaction you experienced with the way your recent claim was handled. Your concerns have now been passed along to __(name of person & title/department/etc.)__.
Our goal at __(Name of Organization)__ is to handle claims fairly and amicably. Please be assured that all the concerns you have raised will be thoroughly investigated. You can expect to hear back from __(name of person & title/department/etc.)__ __(shortly/within the next # weeks/etc.)__.
Yours truly,