[tambre]

What year was this? While I can't find a source I believe Gmail has supported IPv6 for sending and receiving since the World IPv6 day back in 2011. I've certainly been doing it since 2017.

Your issue might be rather that Gmail actually enforces all their guidelines on IPv6 instead of silently degrading your reputation behind the scenes like they do for IPv4. So proper RDNS, SPF and DKIM are tablestakes with DMARC and MTA-STS strongly recommended.

[msk-lywenn]

This was maybe three four years ago. That might be it. I lack rDNS but I have everything else. Except for MTA-STS, I’ll check that out

[tambre]

Yeah, rDNS is a hard requirement for IPv6. I believe you should get a hard reject for missing that with a pointer to the documentation.