DNS/用語/wildcards/RFC1912について、ここに記述してください。 

2.7 Wildcard records

   Wildcard MXs are useful mostly for non IP-connected sites.  A common
   mistake is thinking that a wildcard MX for a zone will apply to all
   hosts in the zone.  A wildcard MX will apply only to names in the
   zone which aren't listed in the DNS at all.  e.g.,

           podunk.xx.      IN      NS      ns1
                           IN      NS      ns2
           mary            IN      A       1.2.3.4
           *.podunk.xx.    IN      MX      5 sue

   Mail for mary.podunk.xx will be sent to itself for delivery.  Only
   mail for jane.podunk.xx or any hosts you don't see above will be sent
   to the MX.  For most Internet sites, wildcard MX records are not
   useful.  You need to put explicit MX records on every host.

上の記述は不正確である。 

Wildcard MXs can be bad,
 because they make some operations succeed when they should fail instead.  
Consider the case where someone in
   the domain "widget.com" tries to send mail to "joe@larry".  If the
   host "larry" doesn't actually exist, the mail should in fact bounce
   immediately.  But because of domain searching the address gets
   resolved to "larry.widget.com", and because of the wildcard MX this
   is a valid address according to DNS.  Or perhaps someone simply made
   a typo in the hostname portion of the address.  The mail message then
   gets routed to the mail host, which then rejects the mail with
   strange error messages like "I refuse to talk to myself" or "Local
   configuration error".

Wildcard MX records are good for 
 when you have a large number of hosts which are not directly Internet-connected
 (for example, behind a firewall) and for
 administrative or political reasons it is too difficult 
 to have individual MX records for every host,
 or to force all e-mail addresses to be "hidden" behind one or more domain names.

 In that case, you must divide your DNS into two parts, an internal
   DNS, and an external DNS.  The external DNS will have only a few
   hosts and explicit MX records, and one or more wildcard MXs for each
   internal domain.  Internally the DNS will be complete, with all
   explicit MX records and no wildcards.