Any active domain that's added in a hosting account features a number of records that enable it to perform properly. In the standard case, the name servers (NS records) show where the Internet domain is hosted and they're set on the side of the registrar company, but on the lower level there are certain records set through the hosting company such as A (IP address of the website/server), MX (e-mail server), SRV (using a domain name for non-website purposes on a specific port number such as Voice-over-IP), and so on. Having access to these records and by having the capacity to edit them, you can have your web site with company A and your e-mails with company B, for illustration. This offers you more freedom because you can select the best provider for each of these services and you do not have to stick with only one company. When you select a hosting company, you have to make sure that you have access to all DNS records freely because some companies have an extra fee for this feature.
