How to configure Domino server to automatically route to backup

Hello, guys,
We use two identical F-Secure messaging gateway servers but only one is used. The other server is used only when there is a system crash or automatic fail over of device. The two servers have different IP addresses, but they have identical configuration. My question is how do I configure Domino server to automatically route SMTP mail to backup server if standard server is down.
Thanks in advance.
