[Exim] Exim, store and forward.

This is probably an FAQ, but the answer doesn't seem to be evident on the
website.

Say I was a large ISP and I wished to used exim to store and forward mail
for intermittently connected hosts. The FAQ suggests:

"Large ISPs who provide this kind of functionality do not usually leave
waiting mail on the MTA's queue. Instead, they get it delivered into
per-host directories, one message per file, in one of the special formats
(BSMTP, maildir, or mailstore) and when an ETRN arrives, it kicks off some
completely different program that establishes an SMTP connection to the
host and shovels the waiting mail down it".

Emminently sensible advice. However, is there a sample config for how
you might implement this?

Two approaches come to mind.

The first is to have all the domains in question be treated as local,
causing a delivery to disc in some suitable location. Then the ETRN
mechanism can be used to sweep up these mails and delivery them, directly
via smtp, to a configured mailserver.

The downside of this approach is that this configuration needs to be kept
up to date independantly of the DNS.

A second approach would be to reinject these messages into the exim queue
and have it deliver them to the primary MX. To do this, however, would
mean configuring exim to deliver to the primary MX if there, and to do
local delivery if it is not. Whether and how this is possible is beyond
my current understanding of Exim.

Is it?

Thanks

John