Configure postfix as a backup for Exchange

I want to use postfix on a virtual server as a backup incase my Exchange is down. Postfix should accept every mail and deliver it to the Exchange server instantly or wait until the Exchange server is available.

Existing postfix installation

Existing Exchange installation

Login to your virtual server via ssh.

Create and open the file /etc/postfix/transport

Insert domain.com smtp:exchangehost.name

Run postmap /etc/postfix/transport

Edit /etc/postfix/main.cf

Insert relay_domains = domain.com

Run /etc/init.d/postfix reload