sendmail, 4th Edition

MX Must Point to Host with an A or AAAA Record

The A and AAAA records for a host are lines that give the host’s IP address or addresses:

hostC  IN     A      123.45.67.8                 ← IPv4
hostC  IN     AAAA   3ffe:8050:201:1860:42::1    ← IPv6

Here, hostC is the host’s name. The IN says this is an Internet-type record. The A marks this as an IPv4 A record, with the IP address 123.45.67.8. The AAAA marks this as an IPv6 AAAA record, with the IP address 3ffe:8050:201:1860:42::1.

An MX record must point to a hostname that has an A or AAAA record. To illustrate, consider the following:

hostA  IN     MX  10 hostB           ← illegal
       IN     MX  20 hostC
hostB  IN     MX  10 hostC
hostC  IN     A   123.45.67.8

Note that hostB lacks an A record but hostC has one. It is illegal to point an MX record at a host that lacks an A or AAAA record. Therefore, the first line in the preceding example is illegal, whereas the second line is legal.

Although such a mistake is difficult to make when maintaining your own domain tables, it can easily happen if you rely on a name server in someone else’s domain, as shown here:

hostA    IN     MX  10 mail.other.domain.

The other administrator might, for example, retire the machine mail and replace its A record with an MX record that points to a different machine. Unless you are notified of the change, your MX record will suddenly become illegal.

Note that although an MX record must point to a hostname that has an A or AAAA record, it is illegal for an MX record to point directly to an A or AAAA record:

hostA  IN     MX  10 123.45.67.89           ← illegal ...