You should always write a letter of appreciation for sponsorship after a big event. There may even be more than one sponsor making it necessary for you to write several letters.
In your letter be sure to mention your goals were met, as well as if it was successful or not. Below you will find several, appreciation for sponsorship, example letters to assist you.
First Example:
May 3, 2015
Ms. Emely Sanders
General Manager
Mr. Fisher
Dear Ms. Sanders: