Hi, I also have this problem and stumbled on this forum post from 2023 however I think OP’s original post is different to the issue that his reared it’s head in the last three weeks.
OP was getting…
level=warning msg="[resolver] failed to read from DNS server"
… the new error is…
level=error msg="[resolver] failed to query external DNS server"
I believe I have tracked the new error message to Docker v26.0.0 which dropped three weeks ago.
In particular they made a change to DNS within Docker…
CVE-2024-29018 : Do not forward requests to external DNS servers for a container that is only connected to an ‘internal’ network. Previously, requests were forwarded if the host’s DNS server was running on a loopback address, like systemd’s 127.0.0.53. moby/moby47589
Source: https://docs.docker.com/engine/release-notes/26.0/#bug-fixes-and-enhancements
I have raised a Github issue mailcow/mailcow-dockerized5800 but it seems to just be sitting there amongst the other couple of hundred issues :-(
I believe that those of us on Docker v26.0 no longer have DNS RBL protection for our mailcow instances.
Please go subscribe to that issue if you agree with my conclusion