During the period from November of 1867 and March of 1868, Mark Twain spent most of his time in Washington D.C. and New York City. He had initially accepted the post of Private Secretary for Senator William Stewart, an acquaintance from Nevada. This did not work out so well and he spent much of his time writing correspondence and in arranging to produce his book on the Quaker City excursion.
Frank C. Willson, in an article for The Twainian (Volume 4 no. 4, January 1945) entitled "Twain Spanks a Government Employee for Unofficial Impertinance", writes of one possible reason for Sam's leaving his post with Senator Stewart. This was in regards to one Thomas B. Kirby, Private Secretary to the Post Master General, who objected in a letter to Sam's use of official postal privileges for something other than official use.
There is a devastating reply that Kirby could have made to Mark’s letter — if, after he had written it, — the Courant had chosen to print it. Kirby could have cited Mark to Mark's own article, “My Late Senatorial Secretaryship,” first printed in The Galaxy magazine for May, 1868, and later in several Twain books, including (for easy reference) “Sketches, New and Old,” (1875). In that article Mark reprints four burlesque letters on non-official subjects to which he signs the name of “Mark Twain, for James W. N**. U. S. Senator.” The “N**” of course stands for Nye, but Twain’s actual employer was the current senator from Nevada, William M. Stewart. And, although the four letters are fictional, there is ample reason to believe that the cause of the quick dissolution of the Mark Twain -Stewart secretarial relationship was that Mark took over the task of speaking for his boss much more often and vigorously that did the (presumably squelched but possibly astute) Thomas B. Kirby in behalf of the Post Office Department.