I have had this working before, so I guess it's a simple setup error.
When I send a message from the Postorius interface, it is neither sent out or stored. I briefly see this message: "Your reply has been sent and is being processed.", and then nothing happens.
I have installed maxking's docker-mailman and copied my old setup with basic stuff like passwords and such. The only new thing is that I'm now running in userspace by putting in /etc/subuid and subguid:
mailman:10000:65536
and in /etc/docker/daemon.json:
{ "userns-remap": "mailman" }
So I get the following ownership in /opt/mailman:
drwxrwsr-x 3 10100 10000 4096 nov. 21 09:00 core drwx------ 19 10070 10000 4096 nov. 21 09:27 database drwxr-sr-x 5 10100 10101 4096 nov. 21 09:24 web
(I have created a user named mailman with uid 10100 so, in reality, it says "mailman" there instead of 10100 but I guess that is irrelevant here)