Setting up a CNAME record for any one of the domains or subdomains you've got within a hosting account will permit you to redirect it to a different domain/subdomain. The forwarded domain will lose all its records - A, MX etc, and will take the records of the Internet domain it is being pointed to. In this light, you can't set up a CNAME record to forward your domain to a third-party provider and keep a functional email service with the first hosting company. Also, it is very important to know that a CNAME record is always a string of words rather than a number as it is regularly wrongly identified as the A record of the Internet domain being forwarded. One of the major uses of a CNAME record is to forward a domain name which you own through one provider to the servers of some other company in case you have created a site with the latter. By doing this, the website will appear under your own domain name, not under some subdomain provided by the third-party provider.