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By definition an mx record is: is a type of resource record in the Domain Name System that specifies a mail server responsible for accepting email messages on behalf of a recipient's domain, and a preference value used to prioritize mail delivery if multiple mail servers are available. The set of MX records of a domain name specifies how email should be routed with the Simple Mail Transfer Protocol.
In lamens terms think of how mail gets routed to your home address. You have an address so the mail man goes boom deliver bobs mail here. MX records are like mailmen and the record itself is your address. Take Rackspace MX records for instance:
If your domain is bobwantsmail.com and you email email@example.com and the email is with Rackspace the only way it will deliver to our environment is if the MX record dictates it to do so.
so I mail bob, the mailman record sends it to Rackspace, our mail servers say yes we have the domain bobwantsmail.com and we have a mailbox named bob. Boom bob now has mail.
It does go much more in depth than that but, this is just a simple explanation of MX (mailman) records!