Configure cPanel as backup mail server?

I tried searching for solution but I couldn't find any, I have tried configuring a Backup mail exchanger and setting up my local exchange as primary server and cpanel domain as backup...

I have selected the option backup mail exchanger, I have set up 2 mx records :

Default one for cpanel which was there before exchange "domain.com" with priority 10 and my exchange server "webmail.domain.com" with priority 0.

I'm receiveing mail to exchange server, but when it's down mail gets rejected as "The domain name in the email address is incorrect. Check the address."

I'm sure I missed some steps but I really couldn't find the solution by myself.