Summarize:

Avoid the temptation to begin formulating a response before getting to the very last word in the correspondence. As you read the entire text, other insights may come to mind that will make it much easier to provide a truly constructive response. Let customers immediately know you care for and plan on investigating their concerns, there is personnel responsible for such actions, and a solution will be reached.  Try a few of the following replies to personalize your own:  At (business name), we are 100% committed to meeting the needs of our customers.  One of our customer representatives will reply within 24-hours. In an effort to monitor and improve customer satisfaction at (business name), we would like to inform you we take all feedback seriously, and will reply to your concern as soon as possible. We appreciate your concern and hope to keep your future business.  Please allow us some time to investigate your issue and determine a suitable solution. Whenever possible, go one step further and personally acknowledge the receipt. This is especially important if it will take time to gather the information necessary to adequately respond to customer queries, concerns, or suggestions. There are customer software packages that do this for you, but in general there should be a way to identify the customer, their contact information, and their specific complaint.  Be sure to include enough information, like a unique ID number, in case the customer has more than one bad experience.
Read the customer's email carefully. Acknowledge the customer email as quickly as possible. Track all customer interaction.