All domain names are registered with the central Internet authority that operates the so-called root servers. Your domain name is registered with them, (i.e. Register.com, Network Solutions, DirectNic, etc.) They have a record for your domain that provides the addresses of two or more DNS servers, which are responsible for your domain.
Those DNS servers have several records relative to your domain. They have "host" records, which give IP addresses for named computers within the domain. For example, a couple of host records might look like this:
www.company.com. IN A 127.127.127.127
mail1.company.com. IN A 127.127.127.128
Basically, the www record tells web browsers where to go to find your web site. But the mail is a little more complicated. It depends on the MX record or records. They might look like this:
company.com. MX 10 mail1.company.com.
company.com. MX 10 mail2.company.com.
company.com. MX 20 mail3.company.com.
Those records tell email servers on the Internet where to go when they want to send mail to someone at your domain.