The word “hosting” doesn't describe only one service, but a number of services that offer a variety of functions to a domain name. Having a website and e-mails, as an example, are two individual services although in the general case they come together, so many people see them as one single service. Actually, every single domain name has a number of DNS records called A and MX, which show the server that handles each specific service - the former is a numeric IP address, which defines where the website for the domain name is loaded from, while the latter is an alphanumeric string, which shows the server that deals with 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. Whenever you open a website or send an email, the global DNS servers are contacted to check the name servers that a domain has and the traffic/message is first directed to that company. If you have custom records on their end, the Internet browser request or the email 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 provider and the e-mail messages by another.