The United States did not want to inadvertently recognize the legitimacy of the Confederate States, so it initially refused to establish formal prisoner or mail exchange guidelines. On July 22, 1862, the warring parties signed a prisoner exchange cartel. This not only created a formal mechanism for the exchange of prisoners and mail via flag-of-truce, but had the further effect of emptying the prisons by September 1862. Increasing distrust towards the Confederacy, however, caused the U.S. to unilaterally stop most prisoner exchanges in June 1863, and prison populations began to soar. Accordingly, regular flag-of-truce mail exchanges resumed from July 1863 to the end of the war. The most common exchange point was Old Point Comfort, Virginia, although there were numerous other exchange points during the war as well.
Regulations called for a flag-of-truce letter to be enclosed in an unsealed inner envelope and sent in an outer envelope with postage prepaid to the exchange point. At the exchange point, the outer envelope was discarded, and the letter was examined by military authorities. Delivery from the exchange point to destination required postage of the other side. If the necessary postage stamps had not been attached to the inner envelope by the sender, coins could be attached to or enclosed in the outer envelope to prepay the postage to destination on the inner envelope. Because they were handled only by the postal system of the receiving side, these inner envelopes only show stamps and postal markings of the receiving side.
In many cases, correspondents did not observe the two-envelope regulation. Fortunately, it was also possible to have both sides’ postage on a single envelope if the sender had access to postage stamps of the other side, or if the receiving side allowed a postage due charge for its share of the postage. Envelopes with both U.S. and Confederate postage stamps are particularly prized by collectors.
In general, a POW envelope can be ascribed to a particular prison by a postmark of the nearby city on outgoing mail or by the address on incoming mail. In the case of inner envelopes, outgoing mail can only be attributed to a particular prison by letter contents or by service records of the POW correspondent. Examined markings can also be used to identify a particular prison.
In general, letters addressed to prisons are much scarcer than letters from prisons. Because Confederate prisons apparently limited the amount of correspondence allowed, mail from Confederate prisons is much rarer than mail from Union prisons.