mx

Preference

An MX record or Mail exchanger record is a type of resource record in the Domain Name System (DNS) specifying how Internet e-mail should be routed using the Simple Mail Transfer Protocol (SMTP). Each MX record contains a preference and a host name, so that the collection of MX records for a given domain name point to the servers that should receive e-mail for that domain, and their priority relative to each other.

The A RECORD is the record that points a domain name to an IP.

The MX RECORD is the record that points the email for a domain to mailserver hostname.

By defining priorities in the MX record.