An MX record (Mail eXchange) record defines which server accepts
mail for a particular domain. Also defined in this record is
preferences for the order in which MX’s should be tried. In the past,
having more than one MX record was not a problem, and provided some
failover if the primary MX could not accept mail. As it stands now,
only having one MX record is recommened. The reason for this is that
spammers routinely send mail to a domains secondary MX servers, as
usually secondary mailservers have much less stringent spam and virus
checks. The SMTP protocol has built in retry mechanisms should your
mailserver be unavailable, so unless it is down for an extended period
of time, your mail should not be lost.