Document a warning that disabled IPv6 may lead to an open relay

[jolly-jump]

Hi.

I'm rather opening this in the documentation region since this is rather a docker issue than a mailcow issue.

See: https://community.mailcow.email/d/5013-massenhaft-bounces-durch-spam/8 (german)

The gist is:

• when you have an IPv6 stack it is hard to deactivate it
• the rules documented are not sufficient (in my case)
• As soon as I disable IPv6 as the docs say, the docker stack still creates docker-proxy for IPv6,
• This docker-proxy forwards/nat's everything from IPv6 to the postfix IPv4 Adress,
• Since there is no nat'ing (the IPv6 firewall is disabled) the postfix identifies the request coming from 172.11.1.1
• ...thus automatically accepting mails that are forged to be from my server. -> Open relay 👿

I found several workarounds and i think the best would be to pin the port to the IPv4-address in mailcow.conf like SMTP_PORT=0.0.0.0:25

If no one else has the problem, this ticket can be closed.
The reaction on the thread was minimal, so maybe I'm alone and this is a special case.