This means you can purchase your domain name (e.g., website.com) from Company A, and host your website with Company A, while you host your e-mail accounts for website.com with Company B.
Why would you want to purchase a domain name from say, Company A, and use Company B to host your e-mail accounts for the same domain?
Maybe Company A always has great deals on domain names, and Company B has first-rate and affordable e-mail hosting. This was the case for me.
Follow these steps to configure your hosting accounts:
1. Configure the new e-mail account with your e-mail hosting company
2. Copy the unique MX and CNAME DNS records associated with the e-mail account from your e-mail host
3. Modify the domain name MX and CNAME DNS records with domain name host to point to your e-mail host
Note that the location of MX (or mail exchanger) and CNAME (or canonical name) records vary depending on your hosting company. For this reason, this article is incomplete and requires some work on your part to locate the MX and CNAME DNS records in your hosting packages.
Also note that the MX record update is responsible for routing your mail to the external e-mail host (e-mail server). After you complete this change and it propagates, your mail will flow to the new location.
The CNAME records make it possible for you access the new host via its unique URL for web mail. You should update all e-mail related CNAME records (also called alias records) with the similarly named CNAME record from the external host.