The definition of “hosting” does not describe a single service, but a number of services that offer different functions to a domain name. Having a website and e-mails, for instance, are two separate services even though in the general case they come together, so most of the people see them as one single service. Actually, every domain name has a several DNS records called A and MX, which show the server that manages each particular service - the first one is a numeric IP address, that defines where the website for the domain name is loaded from, while the second one is an alphanumeric string, which shows the server that handles the emails for the domain address. For example, an A record can be 18.104.22.168 and an MX record would be mx1.domain.com. Every time you open a website or send an e-mail, the global DNS servers are contacted to check the name servers that a domain address has and the traffic/message is first directed to that company. When you have custom records on their end, the web browser request or the e-mail will then be sent to the correct server. The reasoning behind working with separate records is that the two services employ different web protocols and you may have your site hosted by one service provider and the e-mail messages by another.