State your appreciation for any customer feedback. (It's just as important to get negative feedback as positive.) If you have taken action as a result of their suggestion, tell them what you've done. Thank them for taking the time to contact you and encourage them to continue.
1234, Main Street
Boston, MA 02123
<Recipient Address Goes Here>
Thank you for taking the time to send us your suggestion <nature of suggestion>. We really appreciate it when our customers take an interest in helping us improve our business.
As with all new ideas, we must first review its feasibility with respect to our business goals and long-term plans. Whether we implement your suggestion in the near term or save it to be reviewed again at a later date, we want to encourage you to keep making suggestions. It's one of the best ways we have of learning how we can better serve you. We look forward to hearing from you again in the future.
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