Returns an associative array containing all the MX records of a specified domain. The name of each element in the array contains the record name. The value contains the preference of the record.
The preference is the order of delivery when one host has multiple MX records. A host can have multiple MX records so that if an attempt to reach one of the gateways is unsuccessful another can be tried. The server with the higher preference number should be contacted only if the servers with lower preference number are unavailable