It's because the postfix docker container is not connected to a docker network that has access to the "lemmy" or "lemmy-ui" container, it's being connected to the "default" docker network. I submitted a pull request for it here that should fix it.
To make it work in the meantime:
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cd into the Lemmy install directory and run
docker compose down -
Edit docker-compose.yml in the same directory, and in the postfix section, put this just below the
postfix:line:networks: - lemmyinternal - lemmyexternalproxy -
Run
docker compose up -d
The indentation of that code is very important. Your postfix section should look like this when it's done:
That should connect the "postfix" container in to a docker network that can communicate with the "lemmy" and "lemmy" UI containers. There's another bug in the default config that doesn't assign a hostname to all the containers, but it doesn't always manifest all the time. You can fix that by making sure each service has a hostname assigned to it, like hostname: lemmy, hostname: lemmy-ui, hostname: postfix etc in the respective service's section of the service: section of the docker compose file.
Fedora isn't the testing distribution for RHEL, CentOS is. Fedora is upstream of CentOS and could be viewed as the bleeding edge in that regard. CentOS used to be downstream of RHEL, but that changed a few years ago when IBM did its first shitty thing at Red Hat. The tree is like:
Fedora (Top of code stream, "unstable" from a business perspective)
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CentOS (midstream, much less frequent feature updates)
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RHEL (end of stream, stable/predictable/reliable/etc)
And I couldn't disagree more about RHEL adding little value. You're not going to run a server on Fedora for something you want/need to rely on, and especially rely on not to change much/cause breaking changes. That's what RHEL is for and it is the gold standard in that regard.
And that's not even mentioning the fact that Red Hat support is some of the absolute best in the world. Motherfuckers will write a bespoke kernel module for you if that's what it takes to fix your issue. Not sure if that's still true after the IBM takeover though, but that was my experience with them before that.