Sir/Madam,
My name is _____ (name) and I am a resident of __________ (mention address). This email is to inform you that recently I visited your restaurant ___________ (name of the restaurant) with my ________ (friends/family/other).
We assumed that your restaurant is known for its quality food and service but unfortunately my ______ (first/other_ experience at your restaurant was bad and I may never visit your restaurant again. _______ (mention points- made a reservation but still had to wait for seating/bad behaviour/food quality issues/other). Furthermore, we did not expect this unprofessional behaviour from you.
Kindly, improve your customer service and if you need to discuss this matter further, you can contact me.
Yours sincerely,
_____________ (Your name),
_____________ (Contact number)