DNS/1/MX/2821について、ここに記述してください。

http://www.sorbs.net/faq/rfc2821.html

5. Address Resolution and Mail Handling 

Once an SMTP client lexically identifies a domain to which mail will
   be delivered for processing (as described in sections 3.6 and 3.7), a
   DNS lookup MUST be performed to resolve the domain name [22]. 
The names are expected to be fully-qualified domain names (FQDNs):
   mechanisms for inferring FQDNs from partial names or local aliases
   are outside of this specification and, due to a history of problems,
   are generally discouraged. 

The lookup first attempts to locate an MX record associated with the name.
  If a CNAME record is found instead, the resulting name is processed as if it were the initial name.
  If no MX records are found, but an A RR is found, the A RR is treated as
   if it was associated with an implicit MX RR, with a preference of 0,pointing to that host.
  If one or more MX RRs are found for a given name,
   SMTP systems MUST NOT utilize any A RRs associated with that
   name unless they are located using the MX RRs; the "implicit MX" rule
   above applies only if there are no MX records present.
  If MX records are present, but none of them are usable, this situation MUST be reported as an error.

   When the lookup succeeds, the mapping can result in a list of
   alternative delivery addresses rather than a single address, because
   of multiple MX records, multihoming, or both.  To provide reliable
   mail transmission, the SMTP client MUST be able to try (and retry)
   each of the relevant addresses in this list in order, until a
   delivery attempt succeeds.  However, there MAY also be a configurable
   limit on the number of alternate addresses that can be tried.  In any
   case, the SMTP client SHOULD try at least two addresses.

   Two types of information is used to rank the host addresses: multiple
   MX records, and multihomed hosts.

   Multiple MX records contain a preference indication that MUST be used
   in sorting (see below).  Lower numbers are more preferred than higher
   ones.  If there are multiple destinations with the same preference
   and there is no clear reason to favor one (e.g., by recognition of an
   easily-reached address), then the sender-SMTP MUST randomize them to
   spread the load across multiple mail exchangers for a specific
   organization.

   The destination host (perhaps taken from the preferred MX record) may
   be multihomed, in which case the domain name resolver will return a
   list of alternative IP addresses.  It is the responsibility of the
   domain name resolver interface to have ordered this list by
   decreasing preference if necessary, and SMTP MUST try them in the
   order presented.

   Although the capability to try multiple alternative addresses is
   required, specific installations may want to limit or disable the use
   of alternative addresses.  The question of whether a sender should
   attempt retries using the different addresses of a multihomed host
   has been controversial.  The main argument for using the multiple
   addresses is that it maximizes the probability of timely delivery,
   and indeed sometimes the probability of any delivery; the counter-
   argument is that it may result in unnecessary resource use.  Note
   that resource use is also strongly determined by the sending strategy
   discussed in section 4.5.4.1.

   If an SMTP server receives a message with a destination for which it
   is a designated Mail eXchanger, it MAY relay the message (potentially
   after having rewritten the MAIL FROM and/or RCPT TO addresses), make
   final delivery of the message, or hand it off using some mechanism
   outside the SMTP-provided transport environment.  Of course, neither
   of the latter require that the list of MX records be examined
   further.

   If it determines that it should relay the message without rewriting
   the address, it MUST sort the MX records to determine candidates for
   delivery.  The records are first ordered by preference, with the
   lowest-numbered records being most preferred.  The relay host MUST
   then inspect the list for any of the names or addresses by which it
   might be known in mail transactions.  If a matching record is found,
   all records at that preference level and higher-numbered ones MUST be
   discarded from consideration.  If there are no records left at that
   point, it is an error condition, and the message MUST be returned as
   undeliverable.  If records do remain, they SHOULD be tried, best
   preference first, as described above.