Interesting. I imagine it may be ips sitting in a pool which are known bad in your case, and in mine known good slightly tipping the black box scales.

I just wish gmail had tech support so I could resolve this directly instead of flailing about trying random obscure solutions.

In my case, the messages from Gmail's MX suggest the problem was reverse DNS for the EHLO name returning the IPv4, for a connection from the IPv6.

I decided I'd need to configure Exim4 to issue a different EHLO name depending on whether it's connecting over IPv4 or IPv6, and then triple check (with tests) that reverse DNS unambiguously matched in each case.

But as deliverability was the priority, disabling IPv6 seemed lower risk than fiddling with config, seeing it work, then finding out a few weeks later that it introduced another "Google mystery factor". Reviewing that is on the todo list somewhere, very low priority.