"Under our reading of your state's election laws, as in effect on July 27, 2020, certain state-law requirements and deadlines appear to be incompatible with the Postal Service's delivery standards and the recommended timeframe noted above. As a result, to the extent that the mail is used to transmit ballots to and from voters, there is a significant risk that, at least in certain circumstances, ballots may be requested in a manner that is consistent with your election rules and returned
promptly, and yet not be returned in time to be counted."
https://www.washingtonpost.com/context/u-s-postal-service-letters-to-states/b50799f2-25ad-40ed-ba1e-9d648b1814ad/?itid=lk_inline_manual_7