Same problem here. I really noticed it only since yesterday.
Additionally the Postfix logs are also full of warnings:
Totally strange, because I haven’t changed anything and DNS resolution is working fine globally and inside the containers.
I just saw in the logs that a Docker update was automatically installed yesterday:
Start-Date: 2023-05-17 04:00:10
Commandline: apt-get dist-upgrade -yq
Upgrade: docker-ce-cli:amd64 (5:23.0.6-1debian.11bullseye, 5:24.0.0-1debian.11bullseye), docker-ce:amd64 (5:23.0.6-1debian.11bullseye, 5:24.0.0-1debian.11bullseye), docker-ce-rootless-extras:amd64 (5:23.0.6-1debian.11bullseye, 5:24.0.0-1debian.11bullseye)
End-Date: 2023-05-17 04:01:07
Exactly since that point it started with the RBL fails. So I guess it is related to this update. I will try to rollback to see if the problem is fixed then.