OK, domain1.com use the 2 nameservers of a cpanel / whm server (let's call it "whmserver"). The nameservers are edited in a way that www.domain1.com and domain1.com points to another linux server (let's call it "nonwhmserver"). Basically A records on zone are set to another IP.
How to setup domain1.com so that domain1.com emails are handled by "whmserver", meaning that account owner can login to "whmserver" cpanel, setup mail accounts ([email protected], [email protected], etc.) and users can use "whmserver"'s webmail function to check their emails?
Edit DNS for the domain in question. You will see the following entry:
mail CNAME yourdomain.com.
Change this to:
mail A xx.xxx.xxx.xx (replace x's with IP of whmserver)
Next.. Find the MX record.
It should look like this:
yourdomain.com. MX yourdomain.com.
You want it to look like this:
yourdomain.com. MX mail.yourdomain.com.
Leave all the other A records that point to 'nonwhmserver' alone.
You need to do the above for every domain that you want mail to stay on the whmserver.
You can tell your clients to go to http://mail.theredomain.com:2095 to access webmail. They will also need to change there incoming and outgoing mail server's in Outlook Express (or whatever they use) to mail.theredomain.com.