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.
We ought not to use arguments or tedious discourses in giving judgement, much less flatteries, but only to pronounce what is known by experience and firm reason. - Cardan Girolamo (1501-1576)