The word “hosting” does not describe just one service, but a set of services that provide a variety of functions to a domain address. Having a site and e-mails, as an example, are two independent services even though in the general case they come together, so most of the people think of them as one single service. Actually, every domain has a several DNS records called A and MX, which show the server that handles each particular service - the first one is a numeric IP address, that specifies where the website for the domain is loaded from, while the second one is an alphanumeric string, which shows the server that manages the emails for the domain name. As an illustration, an A record would be 123.123.123.123 and an MX record is mx1.domain.com. Every time 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. When you have custom records on their end, the 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 work with different web protocols and you may have your website hosted by one provider and the e-mail messages by another.