Thanks for the explanation!Originally Posted by mashamania
Well, in the phrase 'To whom it may concern" whom is used because there's a preposition. If I remember correctly, 'who' cannot be used if there's a prepostion in a sentence. For instance, you cannot say: I saw the guy to who you spoke yesterday. You should say: I saw the guy who you spoke to yesterday OR I saw the guy to whom you spoke yesterday.
Perhaps, I'm wrong because it was a very long time ago that I last saw the rule explaining the difference between 'who' and 'whom'.