First: the phrase should not really be used in a letter to anyone, even if you don't know the name. It should be reserved for very general documents, like general letters of reference, letters to a company as a whole, etc-- when a letter, like a letter of recommendation, is prepared with no known addressee. ) That said, capitalization varies even among reputable sources.
New words, one handy idiom, and a 2-minute quiz — delivered to your inbox to keep your streak alive.