Many mail servers must accept mail from outside systems. There are two main ways that mail can be addressed to a mail server:
Direct addressing ” The mail may be addressed to a user at the mail server computer itself. For instance, if the mail server is mail.threeroomco.com , mail might be addressed to email@example.com . This configuration is simple, because it needs only a normal address (A) record in the domain's DNS server, as described in Chapter 18, Administering a Domain via DNS. The drawback is that the address is longer than it might be.
Domain addressing ” To achieve shorter e-mail addresses and support for backup mail servers, DNS supports a mail exchanger (MX) record. This record tells remote mail servers to send mail to a particular computer if the mail is addressed to a user at the domain. For instance, if the threeroomco.com domain includes an MX record pointing to mail.threeroomco.com , an outside user can address mail to firstname.lastname@example.org , and it will be directed to mail.threeroomco.com . Such a configuration allows for shorter e-mail addresses and allows network administrators to set up backup mail servers (DNS supports multiple MX records). This system is slightly more complex to administer, though.
The "Specifying Addresses and Aliases" section of Chapter 18 describes configuring a DNS server's MX address. If you're setting up a mail server to handle a domain, it's usually best to do it in this way, so you should consult with your DNS system administrator, or read Chapter 18 if you're handling this yourself. In brief, an MX record looks something like this:
@ IN MX 10 mail.threeroomco.com.
This line appears in the domain configuration file, which is normally named after the domain and stored in /var/named . The leading at-sign ( @ ) means that the line refers to the domain itself. The IN column is a standard part of Internet domain name entries. MX , naturally, identifies this as an MX entry. The 10 is the sequence number of the entry. Sending systems try the mail server with the lowest sequence number first, and work their way up if the first mail server doesn't respond. This allows you to set up multiple mail servers, using servers with higher sequence numbers as backup mail servers. Finally, the entry concludes with the complete address of the mail server, including a trailing period ( . ).
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Although outside users may enter a domain name only to send mail to your domain's mail server, internal users must normally specify the complete hostname when configuring an SMTP server name in mail client programs.