Okay, I'm making progress. in our kubernetes config for mailman we're calling a shell script, and in the script is this line: mailman create mailman@mydomain.com --owner=admin@mydomain.com
When I changed that line to read mailman create mailman@foobar.com --owner=admin@foobar.com
and then rebuilt kubernetes, I found that when I hit the API to get all lists, the list mailman@foobar.com existed, and was the default. I could create lists against foobar.com but could no longer create them against mydomain.com.
So it looks like when you're initially configuring mailman you have to create your first mailing list, and whatever domain you use becomes your "primary domain". I don't know where this is documented, but could someone confirm this behavior for me so that I know that I'm on the right path?