relayhost if fail

[listadecorreo]

Hi to all

In the configuration of my main.cf, I have all mail sent to an external 
server (relayhost) I can do to check if the server is operational and if 
it fails to send all mail to another server

relayhost=xxx.xxx.xxx.xxx if fail send to yyy.yyy.yyy.yyy

I'm using postfix 2.5

thanks in advanced