@Orlando seems to be up on this type of thing. I found the 1040V instructions https://www.irs.gov/pub/irs-pdf/f1040v.pdf say 1214, as does https://www.irs.gov/filing/where-to-file-addresses-for-taxpayers-and-tax-professionals-filing-form-1... 

I know sometimes software developers are given a secret PO Box to use. Maybe this is the case and the only reason for the return is that no one was there to sign for the certified mail. 

I would put the envelope in another envelope and send it to the "correct" PO Box now with a note as to what happened. We all know IRS is a very caring agency that will do the right thing, especially in this time of PANDEMIC.