I rebuilt after a (noop) git pull, and now it seems to be working with the second domain added to Postfix’s relay_domains. Before these series of tests, and with the change, I had no more error, but emails did not appear at all, neither in the category nor in the error logs.
In containers/mail-receiver.yml we’ve got:
POSTCONF_relay_domains: forum.example.net, example.net
(Of course, example.net is not what’s actually in the configuration file. What’s in there is the hostname for the forum and the parent domain name, both of which are configured in the DNS)
I noticed that @mpalmer mentioned years ago that adding a second domain could be done but
So I expected the little relay_domains configuration to not be enough, but it seems to work, given that you git pull before rebuilding. There must be some quirk in the way mail-receiver container is built that fails to update pups…