Editing MX record (section)

===The basics===
In the simplest case, a domain may have just one mail server. For example, if an MTA looks up the MX records for example.com, and the DNS server replied with only mail.example.com with a preference number of 50, then the MTA will attempt delivery of the mail to the server listed. In this case, the number 50 could have been any integer permitted by the SMTP specification.

When more than one server is returned for an MX query, the server with the smallest preference number must be tried first.  If there is more than one MX record with the same preference number, all of those must be tried before moving on to lower-priority entries. An SMTP client ''must'' be able to try (and retry) each of the relevant addresses in the list in order, until a delivery attempt succeeds.<ref name="RFC5321">RFC 5321</ref>