Ordinarily, the decision to relay is not based on MX records.
Relaying based on MX records poses a risk that outsiders might use
your server as a relay for their site (that is, they might set up an
MX record pointing to your mail server, and you will relay mail
addressed to them without any prior arrangement).
This relay_based_on_MX feature reverses that
policy. This feature, how it is used, and how it fits into relaying
and spam handling are explained in Section 7.4.4.