This letter informs the customer that you have received his complaint and are working on its resolution. It's important to state when the customer will hear from you again so that he doesn't feel abandoned.
1234, Main Street
Boston, MA 02123
<Recipient Address Goes Here>
This letter is to inform you that we have received your <inquiry/complaint> about <nature of complaint>. We are currently investigating the circumstances surrounding your problem and will have a response for you as soon as we get all the facts. In any case, you will hear from us again, no later than <date>.
Thank you for your patience while we clear up this matter.
encl: <List of enclosed items goes here>