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.
The rising of the Sun and the indiscrete glances interrupt the spirituality of the nocturnal celestial bodies. - Picatrix (Andalusia, ~1000.AD)