Webster's English Dictionary

whom

whom \(')hu:m, u:m\ pn objective case of WHO  : - used as an interrogative 
   or relative; used as object of a verb or a  preceding preposition {to know 
   for ~ the bell tolls -John Donne} or less frequently as the object of a 
   following preposition {the man ~ you wrote to} though now often considered 
   stilted esp. as an interrogative and esp. in oral use; occas. used as 
   predicate nominative with a copulative verb or as subject of a verb esp. in 
   the vicinity of a preposition or a verb of which it might mistakenly be 
   considered the object {~ say ye that I am -Mt 16:15 (AV)} {people ... ~ you 
   never thought would sympathize -Shea Murphy}