Code:
1.1.1.1 = IP associated with the account on the cPanel machine
2.2.2.2 = IP of the website on the non-cPanel server
primarydomain.com. IN A 2.2.2.2
primarydomain.com. IN MX 0 mail.primarydomain.com.
www.primarydomain.com. IN CNAME primarydomain.com.
mail.primarydomain.com. IN A 1.1.1.1
/etc/remotedomains would NOT contain primarydomain.com
/etc/localdomains WOULD contain primarydomain.com
That's how I would do it. http://primarydomain.com and http://www.primarydomain.com would then go to the non-cPanel webserver, mail would go to mail.primarydomain.com (the IP on the cPanel server).
NOTE: a default cPanel-created zone file for a domain looks a little different than that which you see above. cPanel likes to set up primarydomain.com to resolve to the IP of the cPanel server. Then cPanel adds a CNAME for www that references primarydomain.com. Then cPanel adds an MX that points to primarydomain.com.
Of course, if you are using DomainKeys and SPF (Email Authentication in the cPanel interface for that account), there will be some TXT records as well that cPanel will add automatically to the zone.
You may choose to have primarydomain.com and www.primarydomain.com as "A records" pointing to 2.2.2.2 or you may prefer to have them switched. Or you may want primarydomain.com to continue to resolve to the cPanel server and then in the /cpanel interface just set a redirect so that any requests to http://primarydomain.com would be redirected to http://www.primarydomain.com (which would be on 2.2.2.2, the non-cPanel webserver).
Too many possibilities
But what I have listed will work -- you'd have to access the /cpanel interface via http://mail.primarydomain.com/cpanel though, or through the main IP of the cPanel server.
I'm sure the last two paragraphs will add needless confusion.
Mike