How is the preference on an MX record used?
To improve reliability, it is common practice have multiple mail servers for a domain. This requires one MX record for each mail server. The preference determines the order in which the MX records, that is the mail servers, are used. A mailer sending e-mail to the domain will select the MX record with the lowest preference first, and try to connect to that MX record’s mail server. If unsuccessful, the mailer will go through the remaining MX record(s), in ascending order of preference, and try connecting to each corresponding mail server, until successful.
