Google refuses email from IPv6 addresses without reverse name. This restriction sucks, but can be worked around by switching to IPv4 only for Google
I was stuck trying to send emails from root to gmail. I issued a
dpkg-reconfigure postfix, selected “IPv4 only” and this fixed my problem.
I’m still not sure what the real problem was with IPv6. Our domain was hosted on DNSimple and I thought they would take care of the IPv6 reverse DNS stuff. But maybe not.
To be investigated.