Domain Name Service (DNS) is a required part of the mail services infrastructure. sendmail depends on DNS to transmit e-mail outside of internal networks. sendmail looks for DNS MX records to determine which system is used to transmit e-mail to each destination domain. DNS provides host names to the IP address service and serves as a database for mail administration. Networks generally organize their hosts into a hierarchy of administrative domains.
A domain is a directory structure for electronic mail addressing and network address naming. The domain address has the following format.
mailbox@subdomain. . . . . subdomain2.subdomain1.top-level-domain
The part of the address to the left of the @ sign is the local address. The ...