A letter in a dream often carries something you haven't said out loud -- an unspoken truth, an apology, or news you're waiting to receive. An unopened or lost letter can reflect communication left unresolved, something still waiting for the right moment to finally be spoken.
Saturn should be asked for limitation of movements, hiding of secrets, impoverishment of countries, decrease of intentions, temptation of souls and water stagnation. - Picatrix (Andalusia, ~1000.AD)