In order to locate mail servers that handle email for a domain, a special type of resource record is employed, the mail exchange (MX) record. An MTA or mail transfer agent — this is a mail server like Exchange, Postfix ,or Exim, to name a few — does a special DNS query for MX records and is returned a list of all available mail servers for a domain. The MX resource record type has an additional data field for server preference (sometimes referred to as distance or priority) and via this field has an inherent redundancy and ordering built-in. Mail delivery will be attempted first to the MX record with the lowest numerical preference.
The MTA will attempt to connect to and deliver an email message to a server returned in the list of authorized mail servers for a domain. A server will be selected from the list based on the preference value. A lower value will be chosen over higher values. In the event that a lower valued preference is unavailable the next lowest scored server will be chosen. If two servers have an equal preference, a random server will be chosen for mail delivery.
Because of the inherent redundancy in email delivery it is recommended that only Simple Host Records are used to define MX records. While it is not incorrect to use a Load Balanced Host Record it is not necessary.
Create MX records following the steps in Simple Host Records, with the exception of extended host name syntax and the need to mark the record as MX type. The extended syntax is to prefix the host name with a preference number followed by a semi-colon (“;”). Finally select Yes from the Mail Entry dropdown list.
From the above example, two MX records have been created. The first example has been created with a preference of 10, and assigned to come inbound over wan1. The second has a lesser priority, and will be used in the event that wan1 is unavailable. It is important create PTR records for both of these IP addresses if they will be used for outbound email. Keep in mind that inbound and outbound email can be different, even if they are the same server.