IPv6 and Docker userland_proxy

[pini-gh]

Hi,

I'd like to enable IPv6 for my Mailu instance, but reading the related doc I discover this:

Before enabling IPv6 you MUST disable the userland-proxy in your /etc/docker/daemon.json to not create an Open Relay!

Why is this?

I've been carefully migrating all my Docker services to IPv6, and everywhere I read that userland_proxy should be enabled instead. This is the case for docker-mailserver as well. Why would it be different for Mailu? I fail to understand why it would create an open relay.

Thanks in advance.