Apple Mac OS X Server (Administrator’s Guide) - Manual (Page 372)

372 Chapter 9
How Mail Service Uses SSL
The mail service supports secure IMAP connections with mail client software that requests
them. If a mail client requests a Secure Sockets Layer (SSL) connection, the mail service
automatically complies. The mail service still provides non-SSL (unencrypted) connections to
clients that do not request SSL. The mail service does not require any configuration to use
SSL in this manner. The configuration of each mail client determines whether it connects
with SSL or not.
How Mail Service Uses DNS
Before sending an email, your mail service will probably have a Domain Name System (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, your mail service must know the IP address of the destination. The mail service
relies on a DNS service to look up domain names and determine the corresponding IP
addresses. The DNS service may be provided by your Internet service provider (ISP) or by
Mac OS X Server, as explained in Chapter 14, “DNS Service.”
The mail that your mail service receives comes from other servers, and they use DNS to look
up your mail service. DNS is able to find your mail service if you have created a mail
exchange (MX) record for it. Your MX record specifies the name of the computer that
handles mail service for your domain. This computer is known as a mail host. For example,
the MX record for the domain example.com may specify that the name of the mail host is
mail.example.com. If a mail service wants to send mail that’s addressed to
[email protected], the mail service requests the MX record for the domain
example.com and learns that it should actually send the mail to
[email protected].
An MX record can provide redundancy by listing an alternate mail host for a domain. If the
primary mail host is not available, the mail can be sent to the alternate mail host. In fact, 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.