Thanks for your reply, but no i don't want my mail server to be an open relay. Ill try to be clearer:
I want to use the webmail interface of my mail server. This mailserver has no direct connection to the internet, it uses a relay server which delivers/receives all mail to/from our provider (mailscan). I want to use an sme-server in "servergateway mode" with the snort contrib to forward port 443 to my internal mail-server. While testing I found out the clients ip-adress was forwarded and I have to give my mailserver a route to the clients ip via the sme server.
This is obviously not what I want to do for all possible connections.
Again my question can anyone help me to create a nat rule that masks the clients ip-adres to the internal adress of my sme-server so I only have to add one route in my mail server.