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Mail Exchanger or MX lookups are used to determine the MX records associated with a domain.
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MX records, known as Mail Exchanger records are used to identifiy which server should be used when sending emails to an address at the domain that the records are for.
An example MX record may look like the following:
example.com represents the domain of the record.
MX is the record type.
10 is the priority of the record. The lower the value, the higher the priority.
mail1.example.com is the value of the record. This is the address of the server which should handle email for this domain.
3600 is the TTL (time to live) of the record in seconds, this example represents 1 hour. This means that when a record has had updates made to it, then it will take 1 hour to update.
In many cases multiple MX records are added to a domain name, this allows for primary and secondary (backup) mail servers as well as a form of load balancing.
Lower priority records are always tried first when attempting to deliver mail. In the case that the mail server specified in the record with the lower value returns an error when attemping to deliver mail, the next highest priority MX record will be used. If multiple records have the same priority, then one will be chosen at random as a means to spread the load over multiple email servers.
Backup MX records allow you to set a priority of which mail server should try and process the email first.
The lower the value, the higher the priority.
In this example, the mail server at
mail1.example.com will try to handle processing the email first.
Multiple MX records with the same priority value allow you to have multiple mail servers processing email where one is picked at random. This is useful for spreading the load across many servers or network devices where a single server may not have the capacity to handle all emails at once.
In this example, one of the two mail servers will get a chance to handle processing the emails equally.
A combination of backup and load balanced email servers can also be achieved with additional MX records.
In this example, the mail servers
mail2.example.com will get a chance to handle processing the emails equally. If they fail to respond then the mail server at
mail3.example.com will try to handle processing the email.