Acknowledge that a problem occurred but make it clear that you are not at fault. Be as specific as possible about where the problem lies, and offer to help where you can. Thank them for their business.
1234, Main Street
Boston, MA 02123
<Recipient Address Goes Here>
We were sorry to hear about <nature of problem>, however, the problem was not caused at our end. We believe the responsibility for <nature of problem> lies with <who is responsible>. <Optional additional details.>
I know problems can be frustrating and I will be happy to help you if I can. <Specific help.> We value your business and look forward to working with you in the future.
encl: <List of enclosed items goes here>