It's not necessary to use valid ip's in the dmz, but if you want to do...

check the correct network masks on the interfaces WAN and DMZ, and
the network mask on the router interface too.

the ip 212.x.x.123 it's in the 212.x.x.112  network less or equal than /29
range

>we've looked around in the mailing list, but didn't find any solution for
our problem:
>
>our system has three interfaces:
>WAN (public IP - 212.x.x.113)
>LAN  (local IP   - 198.168.0.1)
>DMZ  (public IP - 212.x.x.123)
>
>
>the DMZ subnet has the web and email server with IP 212.x.x.123.