Because the second copy of my 2020 return was mailed to the IRS by certified mail I know they received it on 3/1. I'm assuming they have too much of a backlog to deal with it and consequently the AGI doesn't match when I try to Efile. I may just wait a week or two and try to efile again and see if the IRS has discovered my return and my AGI for 2020 will match. Otherwise I'll just have to print and mail (certified)!!
