We have a very odd problem with an SBS 2011 server system.
We have a ZyXel USG-50 firewall connected to two ADSL lines. Line 1 works perfectly, and SMTP, PPTP etc route inbound to the server.
However WAN 2 is configured with identical rules but does not work properly. If we disable WAN 1 interface then we can connect inbound using SMTP, VPN, FTP, whatever we like. As soon as WAN 1 is enabled, the firewall reports ACCESS FORWARD in the logs when we try to VPN on WAN 2, but nothing appears to hit the server. It seems that we have a NAT issue but I am at a loss on this.
Can anyone throw some ideas this way? I have tried everything I can think of!