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Mail Domain
Administration
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 jennie@mail.threeroomco.com . This configuration is simple, because it needs only a normal
address (A) record in the domain's DNS server, as described in
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 jennie@threeroomco.com ,
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 "
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
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 ( . ).NOTE
align=left border=0> 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.
