I had the same issue with mine, was able to resolve it. It’s due to the docker DNS (Unbound) expecting bare metal when going about trying to resolve names/ip’s, but in this case, if you’re running in a virtualised environment, the NAT rears it’s ugly head.
Easy way to think is it’s like a maze, you want to find the exit, but there’s no arrows or indicators. The traffic is trying to get out and find it’s way to google, but it can’t. You need to be able to find a way to modify the unbound server to give both of these:
a) Check IP Ranges listed to access, see if the range your Mailcow server is running on, is also listed. If not, you need to add it
b) Forward DNS Server. This then changes Unbound from trying to resolve addresses itself, to looking to someone else to do it for it
It’s the same way i got mine working fine. No issues. It’s also non standard, but hey, it ain’t stupid if it works 🙂