Can you send email from that system at all, starting with outside the docker container? Can it receive email from an external site? (Does it need to?)
What’s in the mail logs?
You need at least outbound email to set up users and their passwords, not sure if inbound has to work as long as there’s an address to reach the administrator or contact.
Email administration is kind of a tricky beast, I’ve done it for over 30 years and I learn something new nearly every day.
It took me 2 full days to get my discourse testbed server working with email, it only took me 3-4 hours to get the production one going.