The letter can be brief but should outline what in particular was impressive about their performance. The letter should be sent to the people responsible as well as their superiors.
1234, Main Street
Boston, MA 02123
<Recipient Address Goes Here>
We want to thank you for the outstanding job you did <job or service>. <Details of job or service.>
We look forward to working with you again and if you would like to use us as a reference, don't hesitate to ask.
encl: <List of enclosed items goes here>