primary, secondary mailservers

[Len Conrad]

How do I configure in monit?

primary mailserver is x and primary recipient is y

secondary mailserver is z and secondary recipient is w

We don't want to try to send y to z nor w to x.

I need precise config lines because

mailserver = x,z  

doesn't seem to try z ever, like when we use the wrong IP or domain name for x 
to force the failover

thanks
Len