Mail Exchange Record
MX
MX Record

    (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 "denis@foo.co.uk" 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.

   (2007-01-29)