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.