@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.
All friendly creatures are welcome. Be excellent to each other, live humanism, no nazis, no hate speech. Not only for nerds, but the domain is somewhat cool. ;) No bots in general! (only with prior permission) - Registration temporarily closed/approval required, contact me if you want an invite!