At 02:15 23-02-2008, Alessandro Vesely wrote:
An MTA learns if it is part of the MX-list retrieved for delivering
a given message by either comparing MX names to its own canonical
name or checking if an MX IP number belongs to one of its local
interfaces. However, the latter method doesn't work across NATs or
split DNSes. Thus, setting the canonical name in the MX records is
required for reliable operations of backup MXes.
Setting the canonical name in the MX records is not required as you
can always add the domain name to the MTA configuration.
Even if backup MXes are not widely used today, I don't think it
would be a good idea to ban them for good, since there are fault
tolerant functionalities that cannot be obtained otherwise. It is
important to standardize this point clearly because fault tolerance
is obtained by enabling backup MXes on different networks, hence
likely involving different organizations. We'll need that stuff in
case WAN connections suddenly were to crash quite often.
The MX to CNAME restriction is not about a ban on backup MXes.
Regards,
-sm