Scenario:

What is an "MX" record?
What is MX Priority?

Solution:

"MX" ("Mail eXchanger") records are used to specify what server on the Internet is running e-mail software that is configured to handle e-mail for your domain.

An example of DNS MX records is below

MX 10 primary-server.example.com

MX 20 backup-server.example.com

If you are using your own mail server remember to create the matching A record for that server and not a CNAME.

MX Priority

MX records need to have an order of priority on them so that when an email delivery is attempted the message will attempt to go to the server with the highest priority (lowest MX number).  If the message fails delivery it will try the next server on the priority list.

Note: Highest priority mail server is the one with the lowest number in the MX record.  So in the example above primary-server.example.com is the highest priority because its the lowest MX record with a priority of 10.

MX Priority numbers range from 0 to 99.  Typically when creating MX DNS records you would space the priorities out by 10 each time, to allow you flexibility if you need to insert a mail server in the middle.