Apologize for the error and explain what corrective action was taken. It may be worthwhile to ask the customer to confirm that the information you have is correct.
1234, Main Street
Boston, MA 02123
<Recipient Address Goes Here>
We would like to apologize for an error <when and where>. <Optional additional details.>
<Corrective action taken.>.
I hope this will resolve the problem to your satisfaction. If you have additional questions, please feel free to contact <contact name> at <telephone number>. Thank you for your cooperation and patience while we resolve this matter.
encl: <List of enclosed items goes here>