How To Write A Letter To A Corporation . You should send this letter by registered a/d to the addressee so that you will get the acknolwledgement slip by return. When writing a letter of complaint, your best chance of success will come from directing the letter to the customer service department of the company. How to Format and Write a Simple Business Letter from www.thoughtco.com State your intent in the letter’s body text. Method 1writing your complaint letter. If the gender is unknown, use: