Thank You notes are a form of professional business etiquette, and they're based on a genuine desire to express appreciation for work or services to another business. This is a type of 'client relationship' but it's targeted to particularly good and productive situations, and is actually a type of positive reinforcement. Thank you notes can also be directed to particular staff members regarding good work.

Dear (Manager/supervisor/staff member)

I'd like to express my deep appreciation of the excellent service I received from your firm/you/your staff member (name) in our recent (describe business event). The service and advice provided were excellent, timely, and exceptionally efficient.