Efficient written communication, in accordance to the highest standards in this area, is a very important element of building relations of a large organization with clients. It refers particularly to the mail merge communication. In this context large companies identify the need of a deeper analysis of purchasing behavior of consumers, their preferences and ways of perceiving and understanding written letters. Institutional customers have different expectations, less time, other habits than individual customers. The objective of the paper is to identify and adapt key elements of written communication of a large company with institutional clients to their preferences. The paper reviews the available literature about B2B communication and contains the results of a qualitative research conducted by the authors in a cooperation with a large energy company in Poland. As the result, the paper indicates the universal principles of constructing written communications for institutional consumers.