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http://en.wikipedia.org/wiki/U._S._Postmasters
A postmaster is the head of an individual post office. When a postmaster is responsible for an entire mail distribution organization (usually sponsored by a national government), the title of Postmaster General is commonly used. Responsibilities of a postmaster typically include management of a centralized mail distribution facility, establishment of letter carrier routes, supervision of letter carriers and clerks, and enforcement of the organization's rules and procedures.
In the United States, women have served as postmasters since the Revolutionary War and even earlier, under British rule. "Postmaster," regardless of the person's sex, has always been the official title for this position.