I'm not 100% sure how to configure my system.

Our domain is hosted on our cPanel based linux server, we manage our own DNS records.

In our zone record, I changed the "mail.mydomain.com" A record to point to the IP of my Zimbra server.

On the Zimbra server, I configured the domain as "domain.com"

What I want is for the cPanel mail server to act as backup to my Zimbra server. This is where things get confusing because I have to set up the Zimbra server MX record to be lower than the cPanel server. Would the setting be like this?)
0 - mail.mydomain.com
10 - mydomain.com

Or - should I create a new A record for a subdomain, say "zimbra.mydomain.com" and set up my DNS so that:

mail.mydomain.com points to zimbra IP

and MX record is:
0 - zimbra.mydomain.com
10 - mydomain.com

What's confusing me is not knowing how email clients set up to look at "mail.mydomain.com / imap.mydomain.com" and so on will know which server to look for.