(MX Record) A DNS resource record type that says which SMTP server handles electronic mail for a particular domain. E.g. the MX record
foo.co.uk. 1054 IN MX 10 mail.foo.co.uk.
means that mail for an address like "firstname.lastname@example.org" should be sent to "mail.foo.co.uk".
There can be several servers for a domain. The "10" is a priority - the server with the lowest number will be tried first.