The Salamander Letter was a document created by Mark Hofmann in the early 1980s.
The letter was one of hundreds of documents concerning the history of Latter Day Saint movement that surfaced in the early 1980s. The Salamander Letter presented a view of Latter Day Saint founder Joseph Smith's life that stood sharply at odds with the commonly accepted version of the early progression of the church Smith established.
Accepted by some document experts and collectors, and rejected by others, the Salamander Letter generated much discussion and debate inside and outside the Latter Day Saint movement. Kenneth W. Rendell lent credence to it by stating that the ink, paper and postmark were all consistent with the period; he concluded, "[T]here is no indication that the document is a forgery." The document was later demonstrated to be a forgery created by Hofmann, who had been responsible for the "discovery" of many other notable documents. Rendell then recast his conclusion, stating that while there was "the absence of any indication of forgery in the letter itself, there was also no evidence that it was genuine."
More information on the Wikipedia page , which has a transcript of the document and gives details of its impact.