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What is an MX record?

An MX record comprises a FQDN and a priority. The priority is simply a number which is used to choose which mail server to use if multiple MX records exist for a domain name. A mail server trying to send an email to you will always try the lowest number priority first.

Additionally an MX record has a host name to allow email addresses with subdomains. The hostname is usually left blank to specify the mail server for the domain name on its own.

If you have a mail server at mail.somemailserver.com and you want to add this for your domain name mydomain.com, you would add an MX record with a blank hostname, an FQDN of mail.somemailserver.com, and a priority, e.g. 10.
This record would mean that any emails addressed to @mydomain.com would be delivered to the server at mail.somemailserver.com.

If you had a backup mail server that you only wanted mail delivered to in case of a problem with your main mail server, you would add another MX record with an FQDN of mail.someothermailserver.com and a higher priority number( e.g. 20.)
This record would mean that any emails that were addressed to @mydomain.com which could not be delivered to the server at mail.somemailserver.com would be delivered to the server at mail.someothermailserver.com.

Adding a hostname into the MX record simply means that you are specifying the mail servers for a subdomain.
For example adding an MX record for hostname "london", FQDN mail.mylondonmailserver.com with priority 10 would mean that any emails addressed to @london.mydomain.com would be delivered to the mail server mail.mylondonmailserver.com. It would not affect any emails addressed to @mydomain.com.