but a rebellion might slow it down some
About three weeks after federal troops occupied Charleston U.S. mail service had resumed from that city.
From The New-York Times March 7, 1865:
The First Mail from Charleston.
PHILADELPHIA, Monday, March 6.
The steamer Massachusetts, which arrived here to-day, had on board the first United States mail that has left Charleston since the breaking out of the rebellion.
According to a similar photograph at the Library of Congress the image of the Post Office also features “the only remaining palmetto in Charleston”.