MX optimisation question

[Arnt Gulbrandsen <agulbra@xxxxxxxxxxx>]

Suppose, in two years, that an MTA has a 400K MIME mailing list message to
deliver.  Among the recipients are these three addresses:

user1@domain.com
user2@sub.domain.com
user3@ip-provider.net

The MXes are as follows:

domain.com	MX	10 domain.com
		MX	20 ip-provider.net
sub.domain.com	MX	10 sub.domain.com
		MX	20 ip-provider.net
ip-provider.net	MX	10 ip-provider.net

Not too contrived, I think.

The question is, how may I optimise delivery of the message?  I want to
deliver the message to as few machines as possible, in order to use as
little bandwidth as possible, and to level the load peak when delivering
mailing-list messages.

Would it be OK to deliver the message only to ip-provider.net with three
RCPT commands, without trying to connect to domain.com and sub.domain.com?
If the message were addressed only to user1 and user2, would it still be
acceptable to send the message via ip-provider.net?

Would it be OK to deliver the message to domain.com if sub.domain.com and
ip-provider.net were down, with RCPT commands for user1 and user2?  I'm
not too fond of this one, but it only requires a line or two of code :-)

These optimisations all assume that the secondary MXes are closer to the
final destination than the SMTP client is, which seems to be mostly true.

--Arnt