He has not sent to (is it necessary to use the preposition?)us letters since last year.
He has not sent to us letters since last year. -- INCORRECT He has not sent us (any) letters since last year. -- CORRECT He has not sent (any) letters to us since last year.
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He has not sent to us letters since last year. -- INCORRECT
He has not sent us (any) letters since last year. -- CORRECT
He has not sent (any) letters to us since last year. -- CORRECT