Secondary/Backup MX

I'd like to use my cpanel server as a secondary/backup mx server for one of my domains - i.e. if the primary server fails, my cpanel server will accept and hold mail for the domain until the primary server comes back online, and then forward the mail on.

I know how to do this using sendmail, but I am not sure on an exim configuration.

Can anyone help?

 

Ok, I've found part of the answer:

/etc/localdomains
/etc/remotedomains
/etc/relayhosts
/etc/secondarymx

Now, localdomains & remotedomains I've read about on the forums when you want to host a site, but email is hosted elsewhere.

Has anyone used the other two files relayhosts and secondartmx? Is it as simple as putting domain names in these files?

I've also read about the exim option 'manualroute' which some people use to forward mail - which is the best route to use?

 

/etc/relayhosts appears to be a list of users who have temporarily been allowed to relay mail, as they've just checked their mail via pop3 etc.

I can only assume to use /etc/secondarymx?