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.
Sol and Mars are fiery; the heat of Mars is destructive, that of the Sun answers the vivifying heavenly fire. - William Lilly (1602.-1681.)