At 11:34 -0800 on 02/23/2013, Ned Freed wrote about Re: [ietf-smtp]
Reject messages on backup mail exchangers w:
> Multiple incoming mail servers (Mail eXchangers; MX) may be configured
for a DNS domain. The MX with the highest priority, that is, the
*lowest* preference number, MUST be contacted ([RFC 5321]) by the
mailserver of another domain if that other mailserver wants to send mail.
That MUST necessarily has to be interpreted in the context of the SMTP
client. That context may be entirely different from the context the
mail exchanger is operating in. More specifically, it's entirely possible
for a mail exchanger providing secondary MX to be able to see the primary
MX but for the client connected to the secondary not to be able to. There
are also a myriad of configurations that could lead a client to fail
on sending
to a primary in such a way that it will fall back to the secondary. Indeed,
there are many sites out there that depend on such configurations.
There is also the issue of Internet Facing DNS Servers vs Local DNS
Servers. The Local DNS Server that is being accessed by the mail
exchanger (ie: Secondary MX) may have a different MX list then the
one being used by the Mail Client or sending SMTP MTA Server. This
can mess up any assumption that the MX lists are the same to everyone.
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