Correspondence may be external and internal. External correspondence is that between the organization and the outsiders (other firms, organizations, companies etc.) that is to say, all the letters received from and sent out to other firms come under external correspondence.
Internal correspondence means the correspondence between the departments of the company, between branches and head office of the same organization. Branches are also considered as a department of the organization and therefore, the correspondence is considered as an internal one.
The main object of correspondence is gained through an efficient and economic communication service. Every letter is a salesman. Every letter carries a certain message. It is a silent ambassador. It attracts the people. It enhances the reputation of the firm.