This doesn't answer your questions but did come to mind as a mention of mail crossing enemy lines (from Boggy Depot to Benton County AR). Phillips calls this a "rebel mail" so it may have been purely military correspondence.
Report of Col. William A. Phillips, Third Indian Home Guard
HDQRS. THIRD BRIG., THIRD DIV., SEVENTEENTH ARMY CORPS,
Fort Gibson, C. N., April 25, 1865.
SIR: The first rebel scout, consisting of eighteen men, trying to push through to Benton County, Ark., with a rebel mail, was met by a scouting party from this place on Snake Creek, fifty-eighth miles west-southwest, on the 23d instant. A company of infantry was some miles behind them. The scouts killed three rebels, left dead on the field; they suppose they wounded several others. The rebel mail was captured, and the fragments of the party driven back again. The rebels left Boggy Depot on the 16th. One citizen scout was shot above the knee, not dangerously. Grand River has been very high; is still high, though falling. Verdigris is also up. As the only way of crossing Arkansas River at present is high up, I am watching my right flank closely. In addition to the troops and scouts already out there, I sent off two other companies, under Capt. Anderson, last night. I have instructed them to watch the river as far as the Cimarron.
Very respectfully, your obedient servant,
W. A. PHILLIPS,
Cmdg. Third Division, Seventh Army Corps.
Official Records Series I. Vol. 48. Part I, Page 202