Qmail as the main email server

I don't really understand what you're looking for. If you want to replace exim with qmail, you can do so, but you'll have to disable all of the mail and webmail related functions within cPanel as your users will no longer be able to use them, obviously. You can disable exim in the WHM > Service Manager. TBH, if you want to use another MTA for some reason you're far better off using a separate mail server since exim is the MTA of choice for cPanel.

 

Ah, OK.

To do that, you would indeed have to modify your MX records, there's no getting around that. If you wanted to use the qmail server in addition to the exim server you could use a smart router in exim to forward email to the qmail server, but that would seem a little pointless. No, the only way to have email delievered to a different server is going to be by modifying the MX records. Also, don't forget to remove the domains from /etc/localdomains and put the in /etc/remotedomains and again disable the Mail functions in cPanel if your not going to have the qmail server forward email onto the cPanel server.