Ooo! Good letter! Mine was shorter: "You're resigning? Who cares."
In cases like this, you might as well be as insulting as possible. "Sir, what is Boston College? Isn't that the community college for local kids that couldn't get into Harvard or BU?"
(I know BC is a lot better than that; it's just about making him feel like as much of a meaningless, insignificant sh*t as possible.)