MX : mail exchange, define a server that process incoming mail
MB : a mailbox domain name (EXPERIMENTAL)
MG : a mail group member (EXPERIMENTAL)
MInfo :mailbox or mail list information
MR : a mail rename domain name (EXPERIMENTAL)
Normally, you need MX record,
for example, your domain is nowhere.net,
and you define a MX record:
MX 10 noserver
then each mail send to noone@nowhere.net
will be actually send to noserver.nowhere.net
if several MX is used, then the 10 will be used to judge
process order. (If a machine of 10 is not on, then
a machine of value large than 10 will be used instead)
