The term ONE SENDING A LETTER can be summarized as a MAILER, which is a person who sends letters. Mailers are responsible for the delivery of physical mail, such as letters, postcards, and packages. They are typically employed by postal services or private companies that specialize in delivering mail. Mailers must ensure that all mail is delivered to the correct address and on time. They may also be responsible for sorting mail, preparing it for delivery, and collecting payments for postage.