Ok, I've found what is the problem. Seems that last night I was too tired to notice this (forgot to mention that here too) - my webserver is behind a firewall using PF.
My public IP which is registered to the domain is assigned to the BSD gateway, which redirects all the external HTTP requests to the internal web server.
webserver -> IP 10.x.x.x
gateway -> IP <nn.ss.hh.hh.> points to mydomain.net
From the internal network I just use the "hosts" file on the systems to assign a specific IPs to a specific hosts - I don't really need a DNS server for this one since the internal network is quite small.
What I was trying to do is to access the webserver from the internal network - after adding an entry to the locals "hosts" files everything is fine. The access from the outside was working too - I just couldn't test it last night, so I guess that was the problem
Thanks again!
EDIT:
When I go to
http://lists.mydomain.net/mailman/listinfo it works, how to make it work when I enter
http://lists.mydomain.net/
Should I use redirecting with .php or .html pages?