MX (Mail Exchanger) RECORD INFORMATION
These are Mail Exchanger records and are used for the delivery of email. They specify the name and relative preference of mail servers.
MX records are in the format of Hostname → Priority → Domain name. (See example below).
From the above example, the host is set to @ by default (See Host description below). The priority is set to 5. This is relative to other MX records with the same host name. The domain name in this case is set to mymailserver.co.uk, this is a valid MX record. If this record was for the domain domainname.co.uk then it would mean that emails would be handled by mymailserver.co.uk.
Host
Host default is an @ character. This will mean the email address will correspond to the actual domain name i.e. @domainname.com. If you put a hostname in i.e. email, then the email address will correspond to @email.domainname.com for example. You can have more than one host, either with the same name or different. If they are the same, you'll need to use priorities.
Priority
Priority sets the priority of the MX record if you have multiple records. 0 being the highest priority. Values must be between 0 and 65535. Mail is delivered to highest priority records first. If that server isn't available, it'll go to the secondary MX record. If you have MX records with the same priority it'll work in a round robin fashion. ***
Domain
Domain can be an alpha-numeric string including hyphens and periods in between strings. The domain must have its own A record in order for the MX to function. mymailserver.co.uk. is valid. If it's not a fully qualified domain name (FQDN), then the domain that the DNS records are for will be appended to the end of the string.
*** Round robin DNS – means that requests will go one of the specified records. For example if there are two MX records with the same priority , mx1.domainname.com and mx2.domainname.com, both with priority 10, then requests will go to mx1 or mx2. The MX record it chooses will depend on which one is returned when each specified request does a lookup. This is a basic technique of load distribution, load balancing, or fault-tolerance.