Hi,
I think your system is using systemd-resolved for DNS lookups; this hands off the real work of resolving to public resolvers, so RBLs will block your lookups. This is a normal setup for a systemd-based distro but is not appropriate for a mail server.
We don't know if systemd-resolved uses public resolvers until he shows us the output of reseolvectl.
Regards Bjoern
