to whom or to who it may concern, which one is appropriate?
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Both phrases are correct. The first one is more formal and is used in the beginning of the letter, while the latter is informal and is used at the end of the letter.
Both of them have different meanings.
to whom it may concern – is always used when the recipient is unknown. Who it may concern is used when the recipient is known, though the name or position of the recipient is not known.