was there a USPS postmark on the whatever contained the check that was sent to you or any other evidence it was not sent until 2021. note that once customer placed it the mail it's your income even if the USPS doesn;t deliver it for months. you would need proof that regardless of the date on the check it wasn't sent to you until 2021. if this is the case, you should first request a corrected 1099. if that fails, you could file form 3949-A with the IRS but you better have proof such as postmarked documents. there is no requirement to file that form. in any case if it is truly 2021 income you would need to list the amount as a return and allowance.
