Reseller: Go to WHM and click Edit DNS zones, under DNS Functions. If you don't feel comfortable, we can help make the changes for you and confirm that your other DNS settings are correct.
Here is some information to help you understand DNS zones.
Your name servers' job is to find the server and correct DNS zone in order to get the following information.
The A record points your domain to an ip address. (e.g. mydomain.com -> ip address)
The CNAME record points your subdomain to another domain name, like www and mail. (e.g. www.domain.com -> domain.com)
The MX records control where your emails are received. If you want your mail hosted on another server, then this is what you will need to change. (e.g. domain.com -> server)