Before the Civil War, there was mail service between all the different states. During the war, could you still send a letter from, say New York to Georgia, or did the post office shut down services to states on the other side?
From this article: http://postalmuseum.si.edu/letterwriting/lw04.html
Although the purpose of stopping mail service to the South was to isolate and corner the Confederate states, some mail still managed to cross the border in what were known as “flag-of-truce” ships. When the Union began blockading southern ports, letters were often carried across the border by blockade runners or routed through foreign ports. While these methods meant that letters often took a long time to reach their intended recipient on the other side of the border, they still allowed friends and families to stay connected as their divided country raged around them.