Setting up a CNAME record for any one of the domain names or subdomains that you've got in a hosting account allows you to direct it to a different domain/subdomain. The forwarded domain will lose all its records - A, MX etc, and will take the records of the domain it's being forwarded to. In this light, you cannot create a CNAME record to redirect your domain name to a third-party provider and maintain a working e-mail service with the first hosting company. It's also very important to know that a CNAME record is always a string of words and not a number as it's commonly wrongly identified as the A record of the domain address being redirected. One of the primary uses of a CNAME record is to direct a domain address you own through one provider to the servers of some other provider assuming you have set up a site with the latter. By doing this, the site will appear under your own domain, not under some subdomain provided by the third-party company.