I have to admit I haven't seen this one before:
The Better Business Bureau has been filed the above mentioned complaint from one of your customers in respect of their business relations with you.
The details of the consumer's concern are contained in enclosed document. Please give attention to this issue and inform us about your opinion as soon as possible.
We kindly ask you to open the COMPLAINT REPORT (attached to this email) to reply on this complaint.
We are looking forward to your prompt response.
Better Business Bureau