If the other organization insists on sticking to older methods, then best practice is for all parties involved to revert to the last commonly and unarguably fully agreed-to method of communication, which is most likely fax in the US.
which is hilarious, since both companies are probably using a fax gateway anyhow. You end up with email-> gateway -> real fax -> gateway -> email. This is somehow easier than configuring a dmarc record.
