Each registered domain name has at least 2 Name Server records that show where it's hosted i.e. by using these records you direct your domain address to the servers of a certain website hosting company. In this way, you've got both your site and your emails handled by the exact same service provider. On the lower level of the Domain Name System (DNS), nevertheless, there are lots of other records, for example A and MX. The first one shows which server manages the site for a given domain name and is always an IP address (18.104.22.168), while the latter indicates which server manages the e-mails and is always an alphanumeric string (mx1.domain.com). As an example, when you type a domain address in your browser, your request is sent through the global DNS system to the provider whose NS records the domain name uses and from there you could be forwarded to the servers of a different service provider in case you have set an IP address of the latter as an A record for your domain. Having different records for the site and the emails means you could have your site and your e-mails with 2 different providers if you wish.