If, for whatever reason, Gmail doesn't like your setup, even if it's 100% according to specs, it's effectively broken. And there's no one to resort to, often no previous relevant search result, because the errors are vague when not silent (you don't get to know the email was hard rejected, i.e. has not even reached the recipient's Spam box).

Your only hope is for your complain to go viral on HN or Twitter and some Googler takes pitty on you.