This got reprinted today in the Daily Mail. Its been around for awhile. Snopes has a thread from 2009 discussing it.
I tend to agree with Snopes that the letter was heavily edited by someone. Maybe even a collaborative effort to support the Freedmen’s Movement for former slaves. The last posting on Snopes is a genealogy of Jourdon Anderson and Colonel P.H. Anderson. They are real people. http://message.snopes.com/showthread.php?t=45660