RFC5321より
http://www.ietf.org/rfc/rfc5321.txt
2.3.5. Domain Names
Only resolvable, fully-qualified domain names (FQDNs) are permitted
when domain names are used in SMTP. In other words, names that can
be resolved to MX RRs or address (i.e., A or AAAA) RRs (as discussed
in Section 5) are permitted, as are CNAME RRs whose targets can be
resolved, in turn, to MX or address RRs.
ということなので、MXじゃなくてもFQDNであれば、AでもAAAAでもCNAMEでもいい
