I have control of DNS on the server (System Administrator), I just don't know what changes to make to the DNS Zone file. I have tried it in the past, but I never could get it to work? Could you tell me what I need to add to the file or what it should look like?
Just modify the existing MX record or add a new MX record as appropriate for the domain and subdomain and enter the appropriate mail server as the mail exchanger. Make sure the mail servers are setup to handle mail for the appropriate domain or subdomain.