Alright I have a CentOS linux web/mail server. That server gets synced with a local mirror on our lan once every hour:
email - *:15
mysql - *:25
website - *:55

I have a SuSE proxy server / gateway (which may later become smoothwall, ipcop, or something else) that forwards port traffic to our primary server.

Let's say the primary server's ip is 192.168.5.100
and the mirrored server is 192.168.5.101

Is there a way to configure the proxy server/gateway that's configured to forward traffic to 192.168.5.100, to automatically forward traffic to 192.168.5.101 if the primary server ever goes down?

Right now if the primary server goes offline I have to manually change the port forwarding on our gateway. If there was a way to automate this, then our site would site and mail server would experience virtually no downtime.

How about a script which does a quick ping of your primary server every 5 minutes. Wouldn't take much server resources if you wanted to do it more frequently. If no response is received, it switches to your mirror based on additional rules you define to adjust the port fowarding. Is a simple bash script taking your existing manual commands, but automated. Next time the gateway receives ping acknowledgement from the primary server, the rules revert.

Check to see if Linux supports VRRP, if so run that on the two hosts behind one virtual ip address, job done