456, X Street, Y Avenue,
[Subject: Normally bold, summarizes the intention of the letter] -Optional-
Dear Jack Martinez,
You have been our janitorial service of choice for the past 5 years. We have always been satisfied with your staff. However, recently, your services have deteriorated and we are no longer satisfied with the job that you do. Although it might be difficult, I regret to inform you that we have decided to use another janitorial agency. We hope that you understand and we will continue to be friends.