I've had similar, unfathomable problems with Gmail delivery for a long time, until I added a PTR record for reverse IP lookup for the IPv6 version of my IP addr

I've had similar, unfathomable problems with Gmail delivery for a long time, until I added a PTR record for reverse IP lookup for the IPv6 version of my IP address. That did the trick, I haven't had delivery trouble since, even without DKIM (but with SPF).