Although if you tend to consistently speak with the "archaic" forms, you end up sounding like a stuffed shirt.
In short, whom is used for a direct or indirect object or for the object of a prepostion, and who is used as a nominative pronoun (such as I or he).
"To whom it may concern" is still widely used in formal letters, but if you ask your mailman "To whom did you give the letter?" you may be looked at strangely.
The same rules apply for whoever and whomever.
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