Plaintiff's have things to prove too - about mailing.

On April 7, 2006, the Appellate Term, Second Department in
Staten Is. Chiropractic Assoc., P.C. v Long Is. Ins. Co.,
2006 NYSlipOp 50588(U),that where plaintiff merely said that it timely mailed the bills to the defendant, where the defendant said it did not receive them, this was inadequate to prove mailing.

Plaintiff's production of an any and all denial did not prove mailing of that particular bill.