Exchange Server Backup

Is there anyway that I can setup a backup exchange server so that incase my primary exchange server goes down we don’t loose much time and get the other server going and user don’t even feel the difference. I would highly appericate if it is possible please provide me details or links at which I can have a look. Do I have to make any changes in firewall for the Backup exchange server to work and get e-mails from the net to and fro.

Thanks
Jasjeet Singh