Then @diekuh need to look into this again 🙂
No. If you reverse-proxy to your mailcow with $host_name and $host_name is unknown to mailcow, that’s a problem. Just set the correct host.
If you need support with that, please check servercow.de. 🙂
I had same problem, it dissapeared after define ADDITIONAL_SERVER_NAMES in mailcow.conf according to documentation https://mailcow.github.io/mailcow-dockerized-docs/u_e-xmpp-enable/