If you handle the correspondence and clerical work for your boss or your company, you are a secretary. One of your duties as a secretary is to open the mail.
Today, the preferred term for the kind of secretary who works in an office is administrative assistant. The noun secretary can also refer to the person who is in charge of an administrative department in the government, like the Secretary of Defense. A secretary can also be a piece of furniture — specifically, a type of writing desk.