In the example sentence, the pronoun 'whom' is an interrogative pronoun, a pronoun that introduces a question.
The interrogative pronouns are: who, whom, what, which, whose.
The pronoun 'whom' is the only objective interrogative pronoun; in the example sentence, 'whom' is the object of the preposition 'for'.
Note: The pronoun 'whom' also functions as a relative pronoun, a pronoun that introduces a relative clause which 'relates' to the noun antecedent.
The relative pronouns are: who, whom, whose, which, that.
Example: The person for whom the package came no longer lives here. (The relative clause 'relates' to the antecedent 'person'.)