[Exim] one domain, multiple mailbox servers

Hello,

I'm in the process of moving our current mail system from sendmail to
imap, and at the same time splitting the current single mailbox server
into two, with option of introducing more mailbox servers at a later
date.

All mail for the local domain comes in on server 1. If the local user is
unknown, then an unknown_transport moves it to the next server. This
works beautifully.

The problem I encounter is when people on server 2 want to mail server 1
people. The obvious thing to do is probably a table of which user is on
what server, and route accordingly. However, I'd like to get away from
large lists as much as possible and am looking for a listless solution.
(Each server hosts +10,000 users.)

Introducing an unknown transport on server 2 back to server 1 would
create a mail loop.

Is there any way I can achieve this routing back without causing a loop?

I guess that a test could be used where if the sender is a local user
and the message has not already been received/handled by server 1, then
the unknown transport could be applied, and otherwise the message would
be bounced back to the sender (unknown user on any local system).
However, I cannot see how to implement such a test.

Thanks for any help/suggestions,

Rolf Habing
Systems and Networking
Canterbury Christ Church University College