Thank customer for order, explain inability to fulfill order and indicate what needs to be provided before order can be filled.
1234, Main Street
Boston, MA 02123
09 April, 2005
<Recipient Address Goes Here>
Thank you for your recent order <reference to order> of <items ordered>. Unfortunately we are unable to process your order at this time because of some missing information. Please indicate <missing information>. As soon as we receive this information we will be happy to complete your order.
We appreciate your business and look forward to serving you in the future.
encl: <List of enclosed items goes here>