I decrypted the letter, but I don't know what to make of it. You found this among thieves in the Alterac foothills? Very strange...$B$BYou should take this to the magistrate of Southshore. Although this letter is cryptic, his is the closest town to Alterac; if a threat is brewing, then he should know.
Bring the Decrypted Letter to Magistrate Maleb in Southshore.