Yesterday's mail brought the Postmaster Commission document shown below. Issued to William Kilby, appointing him the postmaster at Denneysville, Maine. It is dated February 24, 1800. This is the second oldest such document I have been able to add to my collection.
The Postmaster General at the time was Joseph Habersham, who served in this position between 1795 and 1801
William Kilby was the first postmaster appointed at the Denneysville post office, which was established on January 31, 1800, as shown by the following entry from the Letter Books of the Postmaster General
, vol E, page 457.
Denneysville is in Washington County, on the easternmost point of Maine. I've not been able to determine how long William Kilby served as postmaster, but he died in 1829.