@joeyh - yes, several DNS online checker didn't find any issues, everything was ok, but still Google refused to accept the mails.
Sometimes I had the impression that Google tries to resolve the maildomain, not the domain of the smarthost. However, this works fine when delivered via IPv4.
@ij @joeyh In a joy of anti-IPv6 adoption, Google applies different standards for IPv6 and IPv4 mail servers, in particular SPF records are "mandatory" on v6. Wouldn't be surprised if there were more strange requirements.