Sendmail, 3rd Edition

Set Up MX Records

An MX record is simply the method used by DNS to route mail bound for one machine to another instead. An MX record is created by a single line in one of your named(8) files:

hostA    IN      MX 10 hostB

This line says that all mail destined for hostA in your domain should instead be delivered to hostB in your domain. The IN says that this is an Internet-type record, and the 10 is the cost for using this MX record.

An MX record can point to another host or to the original host:

hostA    IN      MX 0 hostA

This line says that mail for hostA will be delivered to hostA. Such records might seem redundant, but they are not because a host can have many MX records (one of which can point to itself):

hostA   IN      MX 0  hostA
        IN      MX 10 hostB

Here, hostA has the lowest cost (0 versus 10 for hostB), so the first delivery attempt will be to hostA. If hostA is unavailable, delivery will be attempted to hostB instead.

Usually, MX records point to hosts inside the same domain. Therefore, managing them does not require the cooperation of others. But it is legal for MX records to point to hosts in different domains:

hostA   IN      MX 0   hostA
        IN      MX 10  host.other.domain.

Here, you must contact the administrator at other.domain and obtain permission before creating this MX record. We cover this concept in more detail when we discuss disaster preparation later in this chapter.

Although MX records are usually straightforward, there can be a few problems associated with them.

MX Must Point to Host With an A or AAAA Record ...