VPS with cpanel/whm clients see ssl warning when visiting webmail

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http://serverfault.com – I have a centos6 VPS with cpanel/whm, I re-sell the webspace to clients that i do IT work for. Im wondering if such a ssl certificate exists that i would be able to stop the warning my clients see when visiting there individual webmail url's like 'webmail.examplesite.com' and 'webmail.thiswebsite.com' I'm getting an ssl certificate for the VPS's domain (vps1.mydomain.com) so that my clients can use a proper ssl connection with email and i'm wondering if i can do anything to fix this in one motion. (HowTos)