Acknowledge the problem and explain what corrective action you intend to take. Express your regret about the problem and set more positive expectations for the future.
1234, Main Street
Boston, MA 02123
<Recipient Address Goes Here>
I'm sorry <explanation of problem>. <Optional cause of problem>.
<Corrective action being taken>.
I apologize for any inconvenience this may have caused. We look forward to meeting your needs in a more timely fashion in the future. Thank you for your cooperation and patience.
encl: <List of enclosed items goes here>