[exim] Mail Routing with Exim and Queue management

I just wanted to solicit some opinions on using a mailrouting/mailstore
system with Exim.

The quick description of what we are doing is we have a group of servers
(mailrouters) that handle inbound mail. These servers do LDAP lookups
to apply certain features to the message and then relay it to a final
mail server for storage (mailstores)

I know from my reading and experience that Exim is great with the 90%
rule (90% of mail gets delivered right away), but what I would like is
some advice and suggestions on a "0%" rule.

I currently having one big issue. The problem is that we have a
mailstore that has been undergoing a severe ammount of load and has
rejected mail for a couple of minutes. This has turned into a fairly
vicious cycle because over that couple of minutes the mailstores have
queued up mail for this server, which they tend to dequeue all at once.
We've switched from sendmail to Exim for the mailrouters (almost
complete) and one difference seems to be that Exim will dequeue all the
mail pending when it decides to dequeue, while sendmail will dequeue it
gradually over the queue window.

I know the obvious solution is to handle the load problem on the
mailstore, but we will create the same situation if/when we do maintence
on a mailstore. In my experience, mailsystems tend to be tests of
extremes. Stress is where the weaknesses show, and I like to fix those
weaknesses as best as I can when I find them.

So that said, and with the understanding that Exim's queue runner is a
very simple system, does anyone have any advice, comments, questions
flames etc etc to offer me?

In the transport definition for delivery to the mailstores, I do have
delay_after_cutoff = false

The mailstores are Redhat boxes running Cyrus IMAP and Sendmail.

Just some notes about future plans I am considering. I'm thinking about
having exim deliver the mail directly to cyrus imap via lmtp on the
remote servers. I don't suppose anyone has done anything like this?


Erik.