E-mail and DNS
The Domain Name System (DNS) plays an important role in e-mailing. The DNS is used by both, e-mail clients and e-mail servers. Even if you do not intend to maintain your own DNS server, a thorough understanding of DNS's role in e-mailing is a necessity for the mail server operator. This section assumes that the reader has basic knowledge of how DNS works in general.
DNS record types used by e-mail applications
In many networking scenarios, only two DNS record types are used—the A record and PTR record. These map hostnames to IP addresses and IP addresses to hostnames respectively. These record types are also used for e-mail, but there is also a third DNS record type that is uniquely available for e-mail.
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