Editing MX record (section)

==="Backup" MX===
Some domains will have several MX records, one of which is intended as a "backup" - with a higher preference number so that it would not normally be picked as the target for email delivery. 

However, in the case of errors from the lower-numbered hosts, (perhaps due to an outage of some sort), sending email servers will deliver to the "backup" host - ''queue.example.com'' in the example below:

{{sxhl|2=zone|
Domain          TTL   Class    Type  Priority   Host
example.com.    1936    IN      MX   10         onemail.example.com.
example.com.    1936    IN      MX   10         twomail.example.com.
example.com.    1936    IN      MX   100        queue.example.com.
}}

If the backup server has direct access to user mailboxes, mail will proceed there, but otherwise will likely be queued on ''queue.example.com'' until the outage is resolved.

In the absence of this sort of arrangement, when a domain's mail servers are all offline, ''sending'' servers are required to queue messages destined for that domain to retry later. However, these sending servers have no way of being notified that a previously offline domain's servers are now available, and so resort to a [[polling system|polling schedule]] - and will only discover that the domain is available whenever they next attempt delivery. The delay between when a receiving domain's servers come online and when delayed messages are finally delivered can be therefore anywhere from minutes to days, depending on the retry schedule of the sending servers - and the receiving domain has no visibility or control over this.