5.1 Locating the Target Host
  Once an SMTP client lexically identifies a domain to which mail will
  be delivered for processing (as described in Sections 2.3.5 and 3.6),
  a DNS lookup MUST be performed to resolve the domain name (RFC 1035
  [2]). 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. Due to a history of
  problems, SMTP servers used for initial submission of messages SHOULD
  NOT make such inferences (Message Submission Servers [18] have
  somewhat more flexibility) and intermediate (relay) SMTP servers MUST
  NOT make them.
  
  The lookup first attempts to locate an MX record associated with the
  name.  If a CNAME record is found, the resulting name is processed as
  if it were the initial name.  If a non-existent domain error is
  returned, this situation MUST be reported as an error.  If a
  temporary error is returned, the message MUST be queued and retried
  later (see Section 4.5.4.1).  If an empty list of MXs is returned,
  the address is treated as if it was associated with an implicit MX
  RR, with a preference of 0, pointing to that host