Veeery interesting. I have a personal domain that forwards to a @gmail.com account. There are several companies I interact with, from banks to retail to just about anything, that send mail to my personal domain that never arrives in my Gmail account, but I can see on the hosted mailserver. For those companies I have to use my @gmail.com address instead for the mail to get through.

Maybe there are more companies than we think that send malformed emails?