Mail service makes use of network services to ensure delivery of mail.
Before sending mail, your mail service will probably have a DNS service determine the Internet Protocol (IP) address of the destination.
The DNS service is necessary because people typically address their outgoing mail by using a domain name, such as example.com, rather than an IP address, such as 198.162.12.12. To send an outgoing message, mail service must know the IP address of the destination. Mail service relies on a DNS service to look up domain names and determine the corresponding IP addresses. The DNS service can be provided by your ISP or by OS X Server.
Additionally, a mail exchange (MX) record can provide redundancy by listing an alternate mail host for a domain. If the primary mail host isn’t available, the mail can be sent to the alternate mail host. An MX record can list several mail hosts, each with a priority number. If the lowest priority host is busy, mail can be sent to the host with the next lowest priority, and so on.
Without a properly configured MX record in DNS, mail might not reach your intended server.
This is how mail service uses DNS:
The sending server reads the mail recipient’s domain name (what comes after the @ in the To address).
The sending server looks up the MX record for that domain name to find the receiving server.
If the MX record is found, the message is sent to the receiving server.
If the lookup fails to find an MX record for the domain name, the sending server assumes that the receiving server has the same name as the domain name, so the sending server does an Address (A) lookup on that domain name and attempts to send the file there.