I and EXPERT SusanY1 above disagree with your sentence, "In that case the remainder of the taxpayer's return could be e-filed." SusanY1 and the instructions for form 8621 (that you cite) both say that you should *not* efile your Form 1020 and separately mail your Form 8621. The two forms should be mailed *together* to the address where you normally mail your 1040.
